Yealink SIP-T2XP Use and care manual

Copyright © 2014 YEALINK NETWORK TECHNOLOGY
Copyright © 2014 Yealink Network Technology CO., LTD. All rights reserved. No parts of this
publication may be reproduced or transmitted in any form or by any means, electronic or
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its consent to downloading and printing copies of the content provided in this file only for private
use but not for redistribution. No parts of this publication may be subject to alteration,
modification or commercial use. Yealink Network Technology CO., LTD. will not be liable for any
damages arising from use of an illegally modified or altered publication.
THE SPECIFICATIONS AND INFORMATION REGARDING THE PRODUCTS IN THIS GUIDE ARE
SUBJECT TO CHANGE WITHOUT NOTICE. ALL STATEMENTS, INFORMATION, AND
RECOMMENDATIONS IN THIS GUIDE ARE BELIEVED TO BE ACCURATE AND PRESENTED
WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED. USERS MUST TAKE FULL
RESPONSIBILITY FOR THEIR APPLICATION OF PRODUCTS.
YEALINK NETWORK TECHNOLOGY CO., LTD. MAKES NO WARRANTY OF ANY KIND WITH
REGARD TO THIS GUIDE, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Yealink Network Technology
CO., LTD. shall not be liable for errors contained herein nor for incidental or consequential
damages in connection with the furnishing, performance, or use of this guide.
Hereby, Yealink Network Technology CO., LTD. declares that this phone is in conformity
with the essential requirements and other relevant provisions of the CE, FCC.
This device is marked with the CE mark in compliance with EC Directives 2006/95/EC and 2004/108/EC.
This device is compliant with Part 15 of the FCC Rules. Operation is subject to the following two conditions:
1. This device may not cause harmful interference.
2. This device must accept any interference received, including interference that may cause undesired
operation.

Note: This device is tested and complies with the limits for a Class B digital device, pursuant to Part 15 of the
FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a
residential installation. This equipment generates, uses, and can radiate radio frequency energy and, if not
installed and used in accordance with the instructions, may cause harmful interference to radio
communications. However, there is no guarantee that interference will not occur in a particular installation. If
this equipment does cause harmful interference to radio or television reception, which can be determined
by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more
of the following measures:
1. Reorient or relocate the receiving antenna.
2. Increase the separation between the equipment and receiver.
3. Connect the equipment into an outlet on a circuit different from that to which the receiver is connected.
4. Consult the dealer or an experience radio/TV technician for help.
To avoid the potential effects on the environment and human health as a result of the
presence of hazardous substances in electrical and electronic equipment, end users of
electrical and electronic equipment should understand the meaning of the crossed-out
wheeled bin symbol. Do not dispose of WEEE as unsorted municipal waste and have to
collect such WEEE separately.
We are striving to improve our documentation quality and we appreciate your feedback. Email
your opinions and comments to [email protected].

Yealink IP phone firmware contains third-party software under the GNU General Public License (GPL).
Yealink uses software under the specific terms of the GPL. Please refer to the GPL for the exact terms and
conditions of the license.
The original GPL license, source code of components licensed under GPL and used in Yealink products can
be downloaded from Yealink web site:
http://www.yealink.com/GPLOpenSource.aspx?BaseInfoCateId=293&NewsCateId=293&CateId=293.

About This Guide
v
This guide is intended for administrators who need to properly configure, customize,
manage, and troubleshoot the IP phone system rather than end-users. It provides
details on the functionality and configuration of IP phones.
Many of the features described in this guide involve network settings, which could affect
the IP phone’s performance in the network. So an understanding of IP networking and a
prior knowledge of IP telephony concepts are necessary.
This guide covers SIP-T28P, SIP-T26P, SIP-T22P and SIP-T20P IP phones. The following related
documents are available:
Quick Start Guides, which describe how to assemble IP phones and configure the
most basic features available on IP phones.
User Guides, which describe the basic and advanced features available on IP
phones.
Auto Provisioning Guide, which describes how to provision IP phones using the
configuration files.
<y0000000000xx>.cfg and <MAC>.cfg template configuration files.
IP Phones Deployment Guide for BroadSoft UC-One Environments, which describes
how to configure BroadSoft features on the BroadWorks web portal and IP phones.
For support or service, please contact your Yealink reseller or go to Yealink Technical
Support online: http://www.yealink.com/Support.aspx.
The information detailed in this guide is applicable to firmware version 73 or higher. The
firmware format is like x.x.x.x.rom. The second x from left must be greater than or equal
to 73 (e.g., the firmware version of SIP-T28P IP phone: 2.73.0.40.rom). This administrator
guide includes the following chapters:
Chapter 1, “Product Overview” describes the SIP components and SIP IP phones.
Chapter 2, “Getting Started” describes how to install and connect IP phones and
the configuration methods.
Chapter 3, “Configuring Basic Features” describes how to configure the basic
features on IP phones.
Chapter 4, “Configuring Advanced Features” describes how to configure the

Administrator’s Guide for SIP-T2xP IP Phones
vi
advanced features on IP phones.
Chapter 5, “Configuring Audio Features” describes how to configure the audio
features on IP phones.
Chapter 6, “Configuring Security Features” describes how to configure the security
features on IP phones.
Chapter 7, “Resource Files” describes the resource files that can be downloaded
by IP phones.
Chapter 9, “Troubleshooting” describes how to troubleshoot IP phones and
provides some common troubleshooting solutions.
Chapter 10, “Appendix” provides the glossary, reference information about IP
phones compliant with RFC 3261, SIP call flows and the sample configuration files.
This section describes the changes to this guide for each release and guide version. For
more information on changes, refer to version-specific release notes of Yealink IP
phones online:
http://www.yealink.com/DocumentDownload.aspx?CateId=142&flag=142.
This version is updated to remove SIP-T21P and SIP-T19P IP phones. The following section
is new for this version:
Hide Features Access Code on page 309
Major updates have occurred to the following sections:
Physical Features of IP Phones on page 4
Configuration Files on page 18
ReCall on page 236
Distinctive Ring Tones on page 264
BLF List on page 303
Static DNS Cache on page 368
Voice Quality Monitoring on page 391
Appendix D: Configuring DSS Key on page 513
Appendix B: Time Zones on page 509
The following sections are new for this version:

About This Guide
vii
Notification Popups on page 54
Call Display on page 62
Input Method Customization on page 99
Off Hook Hot Line Dialing on page 131
Feature Key Synchronization on page 215
BLF List on page 303
Capturing the Current Screen of the Phone on page 355
Voice Quality Monitoring on page 391
Major updates have occurred to the following sections:
Configuration Files on page 18
DHCP on page 22
Configuring Basic Network Parameterson page 22
Upgrading Firmware on page 41
Backlight on page 59
Phone Lock on page 71
Time and Date on page 77
Language on page 89
Anonymous Call Rejection on page 161
DTMF on page 246
Distinctive Ring Tones on page 264
Remote Phone Book on page 277
LDAP on page 281
Message Waiting Indicator on page 310
Multicast Paging on page 316
VLAN on page 380
802.1X Authentication on page 416
Transport Layer Security on page 457
Secure Real-Time Transport Protocol on page 467
Encrypting Configuration Files on page 470
Analyzing Configuration File on page 497
The following sections are new for this version:
Provisioning Server on page 20

Administrator’s Guide for SIP-T2xP IP Phones
viii
Static DNS Cache on page 368
Background Noise Suppression on page 451
Automatic Gain Control on page 451
Major updates have occurred to the following section:
Configuration Files on page 18
Audio Codecs on page 442
Acoustic Clarity Technology on page 450
The following sections are new for this version:
Directory on page 133
Search Source in Dialing on page 135
Major updates have occurred to the following section:
Transport Layer Security on page 457
The following section is new for this version:
Power Indicator LED on page 50
Major updates have occurred to the following sections:
DHCP on page 22
Replace Rule on page 117
Dial-now on page 120
Contrast on page 57
Backlight on page 59
Time and Date on page 77
Key as Send on page 113
Anonymous Call on page 157
LDAP on page 281
Busy Lamp Field on page 293
Action URL on page 335
IPv6 Support on page 430
Transport Layer Security on page 457
Upgrading Firmware on page 41

About This Guide
ix
Resource Files on page 477
Documentations of the newly released SIP-T19P and SIP-T21P IP phones have also been
added.
Major updates have occurred to the following sections:
Action URL on page 335
Action URI on page 351
Major updates have occurred to the following sections:
Logo Customization on page 103
Anonymous Call on page 157
Distinctive Ring Tones on page 264
Server Redundancy on page 356
Transport Layer Security on page 457
Secure Real-Time Transport Protocol on page 467
Encrypting Configuration Files on page 470
Local Contact File on page 484
Viewing Log Files on page 489
Capturing Packets on page 494
Major updates have occurred to the following section:
Appendix B: Time Zones on page 509
Major updates have occurred to the following section:
Appendix D: Configuring DSS Key on page 513

Administrator’s Guide for SIP-T2xP IP Phones
x
The following sections are new for this version:
Hot Desking on page 332
TR-069 Device Management on page 424
IPv6 Support on page 430
Major updates have occurred to the following sections:
Configuring Network Parameters Manually on page 28
Softkey Layout on page 107
Directed Call Pickup on page 219
Distinctive Ring Tones on page 264
Action URL on page 351
Server Redundancy on page 355
VLAN on page 380
Transport Layer Security on page 457
Local Contact File on page 484
The following sections are new for this version:
Configuring Network Parameters Manually on page 28
Contrast on page 57
Backlight on page 59
Logo Customization on page 103
Softkey Layout on page 107
Key as Send on page 113
Call Log on page 137
Live Dialpad on page 145
Auto Answer on page 152
Call Completion on page 155
Anonymous Call on page 157
Anonymous Call Rejection on page 161
Busy Tone Delay on page 175
Return Code When Refuse on page 176
Early Media on page 178

About This Guide
xi
180 Ring Workaround on page 178
Use Outbound Proxy in Dialog on page 179
SIP Session Timer on page 181
Session Timer on page 184
ReCall on page 236
Transfer via DTMF on page 253
Intercom on page 255
Message Waiting Indicator on page 310
Multicast Paging on page 316
Call Recording on page 326
Static DNS Cache on page 368
LLDP on page 376
VLAN on page 380
VPN on page 388
Quality of Service on page 410
Configuring Audio Features on page 439
Secure Real-Time Transport Protocol on page 467
Appendix B: Time Zones on page 509
Major updates have occurred to the following sections:
Dial Plan on page 116
Transport Layer Security on page 457
Encrypting Configuration Files on page 470
Troubleshooting on page 489
The following sections are new for this version:
Dialog Info Call Pickup on page 234
Web Server Type on page 64
Tones on page 270
Hot Desking on page 332
Action URL on page 351
Action URI on page 339
Resource Files on page 477
Appendix D: Configuring DSS Key on page 513

Administrator’s Guide for SIP-T2xP IP Phones
xii
Major updates have occurred to the following sections:
Dial Plan on page 116
Phone Lock on page 71
Time and Date on page 77
Busy Lamp Field on page 293

Table of Contents
xiii
About This Guide ...................................................................... v
Documentations...............................................................................................................................v
In This Guide ....................................................................................................................................v
Summary of Changes ....................................................................................................................vi
Changes for Release 73, Guide Version 73.40...................................................................... vi
Changes for Release 73, Guide Version 73.16...................................................................... vi
Changes for Release 72, Guide Version 72.26..................................................................... vii
Changes for Release 72, Guide Version 72.25.................................................................... viii
Changes for Release 72, Guide Version 72.1...................................................................... viii
Changes for Release 71, Guide Version 71.165.................................................................... ix
Changes for Release 71, Guide Version 71.141.................................................................... ix
Changes for Release 71, Guide Version 71.140.................................................................... ix
Changes for Release 71, Guide Version 71.125.................................................................... ix
Changes for Release 71, Guide Version 71.120.................................................................... ix
Changes for Release 71, Guide Version 71.110.....................................................................x
Changes for Release 70, Guide Version 70............................................................................x
Changes for Release 70, Guide Version 2.0.......................................................................... xi
Table of Contents ...................................................................xiii
Product Overview..................................................................... 1
VoIP Principle....................................................................................................................................1
SIP Components...............................................................................................................................2
SIP IP Phone Models........................................................................................................................3
Physical Features of IP Phones ................................................................................................4
Key Features of IP Phones........................................................................................................8
Getting Started....................................................................... 11
Connecting the IP Phones .............................................................................................................11
Initialization Process Overview .................................................................................................... 14
Verifying Startup ............................................................................................................................15
Reading Icons ................................................................................................................................16
Configuration Methods................................................................................................................. 18
Phone User Interface..............................................................................................................18
Web User Interface ................................................................................................................18
Configuration Files..................................................................................................................18

Administrator’s Guide for SIP-T2xP IP Phones
xiv
Provisioning Server........................................................................................................................ 20
Supported Provisioning Protocols .........................................................................................20
Setting up the Provisioning Server........................................................................................20
Deploying Phones from the Provisioning Server .................................................................21
Configuring Basic Network Parameters ......................................................................................22
DHCP .......................................................................................................................................22
Configuring Network Parameters Manually ........................................................................28
PPPoE .......................................................................................................................................33
Configuring Transmission Methods of the Internet Port and PC Port .................................36
Configuring PC Port Mode.....................................................................................................39
Upgrading Firmware.....................................................................................................................41
Configuring Basic Features .................................................... 49
Power Indicator LED ......................................................................................................................50
Notification Popups........................................................................................................................54
Contrast ..........................................................................................................................................57
Backlight.........................................................................................................................................59
Call Display ....................................................................................................................................62
Web Server Type............................................................................................................................64
User Password ...............................................................................................................................68
Administrator Password ................................................................................................................69
Phone Lock .....................................................................................................................................71
Time and Date ...............................................................................................................................77
Language .......................................................................................................................................89
Loading Language Packs ...................................................................................................... 90
Specifying the Language to Use...........................................................................................97
Input Method Customization ........................................................................................................ 99
Logo Customization.....................................................................................................................103
Softkey Layout..............................................................................................................................107
Key as Send .................................................................................................................................113
Dial Plan........................................................................................................................................116
Replace Rule .........................................................................................................................117
Dial-now ................................................................................................................................120
Area Code.............................................................................................................................124
Block Out ...............................................................................................................................127
Hotline ..........................................................................................................................................129
Off Hook Hot Line Dialing...........................................................................................................131
Directory .......................................................................................................................................133
Search Source in Dialing.............................................................................................................135
Call Log.........................................................................................................................................137
Missed Call Log ...........................................................................................................................138
Local Directory .............................................................................................................................140
Live Dialpad.................................................................................................................................145
Call Waiting.................................................................................................................................. 146

Table of Contents
xv
Auto Redial...................................................................................................................................150
Auto Answer.................................................................................................................................152
Call Completion...........................................................................................................................155
Anonymous Call........................................................................................................................... 157
Anonymous Call Rejection ..........................................................................................................161
Do Not Disturb..............................................................................................................................165
Busy Tone Delay...........................................................................................................................175
Return Code When Refuse ..........................................................................................................176
Early Media.................................................................................................................................. 178
180 Ring Workaround ..................................................................................................................178
Use Outbound Proxy in Dialog...................................................................................................179
SIP Session Timer ......................................................................................................................... 181
Session Timer ...............................................................................................................................184
Call Hold.......................................................................................................................................186
Call Forward ................................................................................................................................191
Call Transfer .................................................................................................................................210
Network Conference ...................................................................................................................213
Feature Key Synchronization......................................................................................................215
Transfer on Conference Hang Up ..............................................................................................217
Directed Call Pickup....................................................................................................................219
Group Call Pickup........................................................................................................................226
Dialog Info Call Pickup................................................................................................................ 234
ReCall............................................................................................................................................236
Call Park .......................................................................................................................................239
Calling Line Identification Presentation..................................................................................... 242
Connected Line Identification Presentation ..............................................................................244
DTMF.............................................................................................................................................246
Suppress DTMF Display .............................................................................................................. 251
Transfer via DTMF ........................................................................................................................253
Intercom........................................................................................................................................255
Outgoing Intercom Calls...................................................................................................... 255
Incoming Intercom Calls ...................................................................................................... 258
Configuring Advanced Features...........................................263
Distinctive Ring Tones ..................................................................................................................264
Tones .............................................................................................................................................270
Remote Phone Book ....................................................................................................................277
LDAP..............................................................................................................................................281
Busy Lamp Field...........................................................................................................................293
BLF List .......................................................................................................................................... 303
Hide Features Access Code.......................................................................................................309
Message Waiting Indicator ........................................................................................................310
Multicast Paging ..........................................................................................................................316
Sending RTP Stream.............................................................................................................316

Administrator’s Guide for SIP-T2xP IP Phones
xvi
Receiving RTP Stream ..........................................................................................................323
Call Recording .............................................................................................................................326
Hot Desking..................................................................................................................................332
Action URL ....................................................................................................................................335
Action URI.....................................................................................................................................351
Capturing the Current Screen of the Phone.......................................................................355
Server Redundancy.....................................................................................................................356
SIP Server Domain Name Resolution.................................................................................. 364
Static DNS Cache ........................................................................................................................368
LLDP...............................................................................................................................................376
VLAN .............................................................................................................................................380
VPN................................................................................................................................................388
Voice Quality Monitoring ............................................................................................................391
RTCP-XR..................................................................................................................................391
VQ-RTCPXR............................................................................................................................392
Quality of Service ........................................................................................................................410
Network Address Translation .....................................................................................................413
802.1X Authentication .................................................................................................................416
TR-069 Device Management ......................................................................................................424
IPv6 Support ................................................................................................................................. 430
Configuring Audio Features ..................................................439
Headset Prior ...............................................................................................................................439
Dual Headset...............................................................................................................................440
Audio Codecs ..............................................................................................................................442
Acoustic Clarity Technology........................................................................................................450
Acoustic Echo Cancellation ................................................................................................. 450
Background Noise Suppression..........................................................................................451
Automatic Gain Control .......................................................................................................451
Voice Activity Detection .......................................................................................................451
Comfort Noise Generation ..................................................................................................453
Jitter Buffer ............................................................................................................................454
Configuring Security Features...............................................457
Transport Layer Security.............................................................................................................. 457
Secure Real-Time Transport Protocol..........................................................................................467
Encrypting Configuration Files ...................................................................................................470
Resource Files........................................................................477
Replace Rule Template ...............................................................................................................478
Dial-now Template.......................................................................................................................479
Softkey Layout Template.............................................................................................................480

Table of Contents
xvii
Directory Template ......................................................................................................................481
Super Search Template...............................................................................................................482
Local Contact File ........................................................................................................................484
Remote XML Phone Book............................................................................................................485
Troubleshooting.....................................................................489
Troubleshooting Methods ...........................................................................................................489
Viewing Log Files..................................................................................................................489
Capturing Packets ................................................................................................................494
Enabling Watch Dog Feature ..............................................................................................495
Getting Information from Status Indicators........................................................................497
Analyzing Configuration File ...............................................................................................497
Troubleshooting Solutions ...........................................................................................................499
Why is the LCD screen blank? .............................................................................................499
Why doesn’t the IP phone get an IP address?...................................................................500
Why does the IP phone display “No Service”? .................................................................500
How do I find the basic information of the IP phone?.......................................................500
Why doesn’t the IP phone upgrade firmware successfully?.............................................500
Why doesn’t the IP phone display time and date correctly? ...........................................500
Why do I get poor sound quality during a call? ................................................................501
What is the difference between a remote phone book and a local phone book? .......501
What is the difference among user name, register name and display name? .............501
How to reboot the IP phone remotely? .............................................................................. 501
Why does the IP phone use DOB format logo file instead of popular BMP, JPG and so on?
................................................................................................................................................502
How to increase or decrease the volume?........................................................................ 502
What will happen if I connect both PoE cable and power adapter? Which has the higher
priority?..................................................................................................................................502
What is auto provisioning? ..................................................................................................502
What is PnP? ..........................................................................................................................502
Why doesn’t the IP phone update the configuration? ......................................................503
What do “on code”and “off code”mean? .......................................................................503
How to solve the IP conflict problem? ................................................................................503
How to reset the IP phone to factory configurations?.......................................................503
How to restore the administrator password? ....................................................................504
What are the main differences among SIP-T28P, IP-T26P, SIP-T22P and SIP-T20P IP phones?
................................................................................................................................................504
Appendix ...............................................................................507
Appendix A: Glossary.................................................................................................................507
Appendix B: Time Zones.............................................................................................................509
Appendix C: Trusted Certificates ...............................................................................................511
Appendix D: Configuring DSS Key.............................................................................................513

Administrator’s Guide for SIP-T2xP IP Phones
xviii
Appendix E: SIP (Session Initiation Protocol) ............................................................................523
RFC and Internet Draft Support ..........................................................................................523
SIP Request............................................................................................................................526
SIP Header ............................................................................................................................ 527
SIP Responses .......................................................................................................................528
SIP Session Description Protocol (SDP) Usage ..................................................................531
Appendix F: SIP Call Flows .........................................................................................................531
Successful Call Setup and Disconnect ...............................................................................532
Unsuccessful Call Setup—Called User is Busy ..................................................................534
Unsuccessful Call Setup—Called User Does Not Answer ................................................536
Successful Call Setup and Call Hold.................................................................................. 538
Successful Call Setup and Call Waiting .............................................................................541
Call Transfer without Consultation ......................................................................................545
Call Transfer with Consultation............................................................................................550
Always Call Forward............................................................................................................555
Busy Call Forward ................................................................................................................558
No Answer Call Forward .....................................................................................................561
Call Conference.................................................................................................................... 564
Index ......................................................................................569

Product Overview
1
This chapter contains the following information about IP phones:
VoIP Principle
SIP Components
SIP IP Phone Models
VoIP
VoIP (Voice over Internet Protocol) is a technology using the Internet Protocol instead of
traditional Public Switch Telephone Network (PSTN) technology for voice
communications.
It is a family of technologies, methodologies, communication protocols, and
transmission techniques for the delivery of voice communications and multimedia
sessions over IP networks. The H.323 and Session Initiation Protocol (SIP) are two
popular VoIP protocols that are found in widespread implementation.
H.323
H.323 is a recommendation from the ITU Telecommunication Standardization Sector
(ITU-T) that defines the protocols to provide audio-visual communication sessions on
any packet network. The H.323 standard addresses call signaling and control,
multimedia transport and control, and bandwidth control for point-to-point and
multi-point conferences.
It is widely implemented by voice and video conference equipment manufacturers, is
used within various Internet real-time applications such as GnuGK and NetMeeting and
is widely deployed by service providers and enterprises for both voice and video
services over IP networks.
SIP
SIP (Session Initiation Protocol) is the Internet Engineering Task Force’s (IETF’s) standard
for multimedia conferencing over IP. It is an ASCII-based, application-layer control
protocol (defined in RFC 3261) that can be used to establish, maintain, and terminate
calls between two or more endpoints. Like other VoIP protocols, SIP is designed to
address functions of signaling and session management within a packet telephony
network. Signaling allows call information to be carried across network boundaries.
Session management provides the ability to control attributes of an end-to-end call.

Administrator’s Guide for SIP-T2xP IP Phones
2
SIP provides capabilities to:
Determine the location of the target endpoint -- SIP supports address resolution,
name mapping, and call redirection.
Determine media capabilities of the target endpoint -- Via Session Description
Protocol (SDP), SIP determines the “lowest level” of common services between
endpoints. Conferences are established using only media capabilities that can be
supported by all endpoints.
Determine the availability of the target endpoint -- A call cannot be completed
because the target endpoint is unavailable, SIP determines whether the called
party is already on the IP phone or does not answer in the allotted number of rings.
It then returns a message indicating why the target endpoint is unavailable.
Establish a session between the origin and target endpoint -- The call can be
completed, SIP establishes a session between endpoints. SIP also supports mid-call
changes, such as the addition of another endpoint to the conference or the change
of a media characteristic or codec.
Handle the transfer and termination of calls -- SIP supports the transfer of calls from
one endpoint to another. During a call transfer, SIP simply establishes a session
between the transferee and a new endpoint (specified by the transferring party)
and terminates the session between the transferee and the transferring party. At
the end of a call, SIP terminates the sessions between all parties.
SIP is a peer-to-peer protocol. The peers in a session are called User Agents (UAs). A
user agent can function as one of following roles:
User Agent Client (UAC) -- A client application that initiates the SIP request.
User Agent Server (UAS) -- A server application that contacts the user when a SIP
request is received and that returns a response on behalf of the user.
User Agent Client (UAC)
The UAC is an application that initiates up to six feasible SIP requests to the UAS. The six
requests issued by the UAC are: INVITE, ACK, OPTIONS, BYE, CANCEL and REGISTER.
When the SIP session is being initiated by the UAC SIP component, the UAC determines
the information essential for the request, which is the protocol, the port and the IP
address of the UAS to which the request is being sent. This information can be dynamic
and will make it challenging to put through a firewall. For this reason, it may be
recommended to open the specific application type on the firewall. The UAC is also
capable of using the information in the request URI to establish the course of the SIP
request to its destination, as the request URI always specifies the host which is essential.
The port and protocol are not always specified by the request URI. Thus if the request
does not specify a port or protocol, a default port or protocol is contacted. It may be