Cisco 7960 Administrator's Manual page 170

Call Flow Scenarios for Failed Calls
Step Action
7
CANCEL (Ring
Timeout)—Gateway 1 to Cisco
SIP IP phone
8
Disconnect—Gateway 1 to PBX
A
9 Release Complete—Gateway 1
to PBX A
10
200 OK—Cisco SIP IP phone to
Gateway 1
11 Release Complete—Gateway 1
to PBX A
Cisco SIP IP Phone 7960 Administrator Guide
B-62
Description
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.
Gateway 1 sends a Disconnect message to PBX A.
Gateway 1 sends a Release Complete message to PBX A
and the call session attempt is terminated.
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.
Gateway 1 sends a Release Complete message to PBX A
and the call session is terminated.
Appendix B SIP Call Flows
78-10497-02