Cisco 7960 Administrator's Manual page 173

Appendix B SIP Call Flows
Step Action
5
4xx/5xx/6xx Failure—Cisco SIP
IP phone to Gateway 1
6 Disconnect—Gateway 1 to PBX
A
7
Release—PBX A to Gateway 1
8
ACK—Gateway 1 to Cisco SIP
IP phone
9 Release Complete—Gateway 1
to PBX A
78-10497-02
Description
The Cisco SIP IP phone sends a class 4xx, 5xx, or class 6xx
failure response to Gateway 1. Depending on which class
the failure response is, the call actions differ.
If the Cisco SIP IP phone sends a class 4xx failure response
(a definite failure response that is a client error), the request
will not be retried without modification.
If the Cisco SIP IP phone sends a class 5xx failure response
(an indefinite failure that is a server error), the request is
not terminated but rather other possible locations are tried.
If the Cisco SIP IP phone sends a class 6xx failure response
(a global error), the search for User B is terminated because
the 6xx response indicates that a server has definite
information about User B, but not for the particular
instance indicated in the Request-URI field. Therefore, all
further searches for this user will fail.
Gateway 1 sends a Release message to PBX A.
PBX A sends a Release message to Gateway 1.
Gateway 1 sends a SIP ACK to the Cisco SIP IP phone. The
ACK 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 attempt is terminated.
Cisco SIP IP Phone 7960 Administrator Guide
Call Flow Scenarios for Failed Calls
B-65