B-47
Cisco SIP IP Phone Administrator Guide
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Figure B-14 Gateway-to-Cisco SIP IP Phone—Called User is Busy
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.
486 Busy Here—Cisco SIP IP
phone to Gateway 1
The Cisco SIP IP phone sends a SIP 486 Busy Here response to Gateway 1. The
486 Busy Here response is a client error response that indicates that User B was
successfully contacted but that User B was not willing or was unable to take the
call.
IP
3. Call Proceeding
6. Disconnect (Busy)
1. Setup
7. Release
PBX A
User A
GW1
IP Network
4. 100 Trying
5. 486 Busy Here
8. ACK
2. INVITE
9. Release Complete
41725
SIP IP Phone
User B