Cisco SIP IP Phone - Cisco SIP IP Phone-To-Cisco SIP IP Phone-Called User Is Busy

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Cisco SIP IP Phone Administrator Guide
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Cisco SIP IP Phone-to-Cisco SIP IP Phone—Called User Is Busy
Figure B-17 illustrates an unsuccessful call in which User A initiates a call to User B but User B is on
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
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. 4xx/5xx/6xx Failure—
Cisco SIP IP phone to Gateway 1
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.
6. Disconnect—Gateway 1 to
PBX A
Gateway 1 sends a Release message to PBX A.
7. Release—PBX A to Gateway 1 PBX A sends a Release message to Gateway 1.
8. ACK—Gateway 1 to Cisco SIP
IP phone
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.
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.
Step Action Description
IP IP
2. 486 Busy Here
3. ACK
1. INVITE B
IP Network
SIP IP
Phone User A
SIP IP
Phone User B
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