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Cisco Unified IP Phone 7960G and 7940G Administration Guide for Release 8.0 (SIP)
OL-7890-01
Appendix B SIP Call Flows
Call Flow Scenarios for Successful Calls
Network Call Forwarding (Busy)
Figure B-12
illustrates successful call forwarding between Cisco SIP IP phones in which User B has
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 Phone B, and, if the line is busy, the call is transferred to
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.
User B requests that if Phone B is busy, the network should forward incoming calls to Phone C.
2.
User A calls User B.
3.
User B is busy.
4.
The network transfers the call to Phone C.
3.
302 Moved Temporarily—SIP
redirect server to SIP proxy
server
The SIP redirect server sends a SIP 302 Moved temporarily message to the SIP
proxy server. The message indicates that User B is not available at Phone B and
includes instructions to locate User B at Phone C.
4.
ACK—SIP proxy server to
redirect server
The SIP proxy server sends the SIP ACK to the SIP redirect server.
5.
INVITE—SIP proxy server to
Phone C
The SIP proxy server sends a SIP INVITE request to Phone C. The request is an
invitation to User C to participate in a call session.
6.
180 Ringing—Phone C to SIP
proxy server
Phone C sends a SIP 180 Ringing response to the SIP proxy server.
7.
200 OK—Phone C to SIP proxy
server
Phone C sends a SIP 200 OK response to the SIP proxy server.
8.
200 OK—SIP proxy server to
Phone A
The SIP proxy server forwards the SIP 200 OK response to Phone A.
9.
ACK—Phone A to SIP proxy
server
Phone A sends a SIP ACK to the SIP proxy server. The ACK confirms that the SIP
proxy server has received the 200 OK response from Phone C.
10.
ACK—SIP proxy server to
Phone C
The SIP proxy server forwards the SIP ACK to the Phone C. The ACK confirms
that Phone A has received the 200 OK response from Phone C.
Step
Action
Description