B-49
Cisco SIP IP Phone Administrator Guide
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Step
Action
Description
1.
Setup—PBX A to Gateway 1
Call Setup is initiated between PBX A and Gateway 1. The Call Setup includes
the standard transactions that take place as User A attempts to call User 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 Cisco SIP IP phone.
In the INVITE request:
•
The IP address of the Cisco SIP IP 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 User 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 Cisco SIP IP phone sends a SIP 100 Trying response to Gateway 1. The
100 Trying response indicates that the INVITE request has been received by the
Cisco SIP IP phone.
5.
180 Ringing—Cisco SIP IP
phone to Gateway 1
The Cisco SIP IP phone sends a SIP 180 Ringing response to Gateway 1. The
180 Ringing 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
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 is terminated.
10.
200 OK—Cisco SIP IP phone to
Gateway 1
The Cisco SIP IP phone sends a SIP 200 OK response to Gateway 1. The 200 OK
response confirms that User A has received the 486 Busy Here response. The call
session attempt is now being terminated.
11.
Release Complete—Gateway 1
to PBX A
Gateway 1 sends a Release Complete message to PBX A and the call session is
terminated.