B-11
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
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.
3.
200 OK—Phone B to Phone A
Phone B sends a SIP 200 OK response to Phone A. The response notifies
Phone A that the connection has been made.
If Phone B supports the media capability advertised in the INVITE message
sent by Phone A, it advertises the intersection of its own and Phone A’s media
capability in the 200 OK response. If Phone B does not support the media
capability advertised by Phone A, it sends back a 400 Bad Request response
with a 304 Warning header field.
4.
ACK—Phone A to Phone B
Phone A sends a SIP ACK to Phone B. The ACK confirms that Phone A has
received the 200 OK response from Phone B.
The ACK might contain a message body with the final session description to
be used by Phone B. If the message body of the ACK is empty, Phone B uses
the session description in the INVITE request.
A two-way RTP channel is established between Phone A and Phone B.
5.
INVITE—Phone B to Phone A
Phone B sends a midcall INVITE to Phone A with new Session Description
Protocol (SDP) attribute parameter.
SDP: a=sendonly
The a= SDP field of the SIP INVITE contains sendonly. This value places the
call on hold.
6.
200 OK—Phone A to Phone B
Phone A sends a SIP 200 OK response to Phone B.
7.
ACK—Phone B to Phone A
Phone B sends a SIP ACK to Phone A. The ACK confirms that Phone B has
received the 200 OK response from Phone A.
The RTP channel between Phone A and Phone B is torn down.
8.
INVITE—Phone B to Phone A
User B takes User A off hold. Phone B sends a SIP INVITE request to Phone A
with the same call ID as the previous INVITE and a new SDP attribute
parameter (sendrecv), which is used to reestablish the call.
9.
200 OK—Phone A to Phone B
Phone A sends a SIP 200 OK response to Phone B.
10.
ACK—Phone B to Phone A
Phone B sends a SIP ACK to Phone A. The ACK confirms that Phone B has
received the 200 OK response from Phone A.
A two-way RTP channel is reestablished between Phone A and Phone B.
Step
Action
Description
