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B-50
Cisco Unified IP Phone 7960G and 7940G Administration Guide for Release 8.0 (SIP)
OL-7890-01
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Client, Server, or Global Error
Figure B-20
illustrates an unsuccessful call in which User A initiates a call to User B and receives a class
4
xx
, 5
xx
, or 6
xx
response.
Step
Action
Description
1.
Setup—PBX A to Gateway 1
Call setup is initiated between PBX A and Gateway 1. Setup includes the
standard transactions that take place as A attempts to call B.
2.
INVITE—Gateway 1 to Cisco SIP IP
phone
Gateway 1 maps the SIP URL phone number to a dial peer. The dial peer
includes the IP address and the port number of the SIP-enabled entity to
contact. Gateway 1 sends a SIP INVITE request to the address it receives
as the dial peer, which, in this scenario, is the IP phone. In the INVITE
request:
•
The IP address of the phone is inserted in the Request-URI field.
•
PBX A is identified as the call session initiator in the From field.
•
A unique numeric identifier is assigned to the call and is inserted in
the Call-ID field.
•
The transaction number within a single call leg is identified in the
CSeq field.
•
The media capability A is ready to receive is specified.
•
The port on which the gateway is prepared to receive the RTP data is
specified.
3.
Call Proceeding—Gateway 1 to PBX A
Gateway 1 sends a Call Proceeding message to PBX A to acknowledge
the Call Setup request.
4.
100 Trying—Cisco SIP IP phone to
Gateway 1
The phone sends a SIP 100 Trying response to Gateway 1. The response
indicates that the INVITE request has been received.
5.
180 Ringing—Cisco SIP IP phone to
Gateway 1
The phone sends a SIP 180 Ringing response to Gateway 1. The response
indicates that the user is being alerted.
6.
Alerting—Gateway 1 to PBX A
Gateway 1 sends an Alert message to PBX A.
7.
CANCEL (Ring Timeout)—Gateway 1 to
Cisco SIP IP phone
Because Gateway 1 did not return an appropriate response within the time
allocated in the INVITE request, Gateway 1 sends a SIP CANCEL
request to the gateway 2. A CANCEL request cancels a pending request
with the same Call-ID, To, From, and CSeq header field values.
8.
Disconnect—Gateway 1 to PBX A
Gateway 1 sends a Disconnect message to PBX A.
9.
Release Complete—Gateway 1 to PBX A Gateway 1 sends a Release Complete message to PBX A, and the call
session attempt terminates.
10.
200 OK—Cisco SIP IP phone to
Gateway 1
The phone sends a SIP 200 OK response to Gateway 1. The response
confirms that A has received the 486 Busy Here response. The call
session attempt terminates.
11.
Release Complete—Gateway 1 to PBX A Gateway 1 sends a Release Complete message to PBX A, and the call
session terminates.