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Appendix B SIP Call Flows
Call Flow Scenarios for Successful Calls
B-20
Cisco SIP IP Phone 7960 Administrator Guide
78-10497-02
Cisco SIP IP Phone-to-Cisco SIP IP Phone Call Hold with
Consultation
Figure B-5 illustrates a successful call between Cisco SIP IP phones in which one
of the participants places the other on hold, calls a third party (consultation), and
then returns to the original call. 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 A calls User B.
2.
User B answers the call.
3.
User B places User A on hold.
4.
User B calls User C.
5.
User B disconnects from User C.
6.
User B takes User A off hold.
7.
The original call continues.
8
INVITE—Cisco SIP IP phone B
to Cisco SIP IP phone A
Cisco SIP IP phone B sends a mid-call INVITE to Cisco
SIP IP phone A with the same call ID as the previous
INVITE and new SDP session parameters (IP address),
which are used to reestablish the call.
Call_ID=1
SDP: c=IN IP4 181.23.250.2
To reestablish the call between phone A and phone B, the
IP address of phone B is inserted into the c= SDP field.
9
200 OK—Cisco SIP IP phone A
to Cisco SIP IP phone B
Cisco SIP IP phone A sends a SIP 200 OK response to
Cisco SIP IP phone B.
10
ACK—Cisco SIP IP phone B to
Cisco SIP IP phone A
Cisco SIP IP phone B sends a SIP ACK to Cisco SIP IP
phone A. The ACK confirms that Cisco SIP IP phone B has
received the 200 OK response from Cisco SIP IP phone A.
A two-way RTP channel is reestablished between IP phone A and IP phone B.
Step Action Description