Cisco Sip Ip Phone-To-Cisco Sip Ip Phone-Called User Is Busy - Cisco SIP IP Phone Administrator's Manual

Appendix B SIP Call Flows
Step Action
3. Call Proceeding—Gateway 1 to
PBX A
4. 100 Trying—Cisco SIP IP phone
to Gateway 1
5. 4xx/5xx/6xx Failure—
Cisco SIP IP phone to Gateway 1
Disconnect—Gateway 1 to
6.
PBX A
Release—PBX A to Gateway 1
7.
8. ACK—Gateway 1 to Cisco SIP
IP phone
9. Release Complete—Gateway 1
to PBX A
Cisco SIP IP Phone-to-Cisco SIP IP Phone—Called User Is Busy
Figure B-17
the phone and is unable or unwilling to take another call.
Figure B-17 Cisco SIP IP Phone-to-Cisco SIP IP Phone—Called User Is Busy
SIP IP
Phone User A
Description
Gateway 1 sends a Call Proceeding message to PBX A to acknowledge the Call
Setup request.
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.
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.
illustrates an unsuccessful call in which User A initiates a call to User B but User B is on
IP
1. INVITE B
2. 486 Busy Here
3. ACK
IP Network
Cisco SIP IP Phone Administrator Guide
Call Flow Scenarios for Failed Calls
SIP IP
Phone User B
IP
B-51