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Appendix B
SIP Call Flows
Cisco SIP IP Phone-to-Cisco SIP IP Phone Network Call Forwarding (Busy)
Figure B-11
requested call forwarding from the network in the event the phone is busy. When User A calls User B,
the SIP proxy server tries to place the call to Cisco SIP IP phone B and, if the line is busy, the call is
transferred to Cisco SIP IP phone C. In this call flow scenario, the end users are User A, User B, and
User C. They are all using Cisco SIP IP phones, which are connected via an IP network.
The call flow scenario is as follows:
1.
2.
3.
4.
Figure B-11 Cisco SIP IP Phone-to-Cisco SIP IP Phone Network Call Forwarding (Busy)
SIP IP
Phone
User A
illustrates successful call forwarding between Cisco SIP IP phones in which User B has
User B requests that if their phone (Cisco SIP IP phone B) is busy, the network should forward
incoming calls to Cisco SIP IP phone C.
User A calls User B.
User B's phone is busy.
The network transfers the call to Cisco SIP IP phone C.
IP
Proxy
Server
1. INVITE B
11. 200 OK
12. ACK
2-way RTP channel
IP Network
Redirect
Server
2. INVITE B
3. 300 Multiple Choices
4. ACK
5. INVITE B
6. 486 Busy Here
7. ACK
8. INVITE C
9. 180 Ringing
10. 200 OK
13. ACK
Call Flow Scenarios for Successful Calls
IP
SIP IP
Phone
User B
Cisco SIP IP Phone Administrator Guide
IP
SIP IP
Phone
User C
B-37
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