Cisco 7960G Administration Manual page 126

Call Flow Scenarios for Successful Calls
Step Action
INVITE—Phone B to Phone C
9.
100 Trying—Phone C to
10.
Phone B
180 Ringing—Phone C to
11.
Phone B
200 OK—Phone C to Phone B
12.
ACK—Phone B to Phone C
13.
A two-way RTP channel is established between Phone B and Phone C.
User B then selects the option to transfer the call to User C.
INVITE—Phone B to Phone C
14.
200 OK—Phone A to Phone B
15.
16. ACK—Phone B to Phone C
17. REFER—Phone B to Phone A
202 ACCEPTED—Phone A to
18.
Phone B
NOTIFY (Event:Refer;
19.
Subscription-State: Active)
Cisco Unified IP Phone 7960G and 7940G Administration Guide for Release 8.0 (SIP)
B-26
Description
Phone B sends a SIP INVITE request to Phone C. The request is an invitation to
User C to participate in a call session.
Phone C sends a SIP 100 Trying response to Phone B. The response indicates that
the INVITE request has been received.
Phone C sends a SIP 180 Ringing response to Phone B.
Phone C sends a SIP 200 OK response to Phone B. The response notifies Phone B
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.
Phone B sends a SIP ACK to Phone C. The ACK confirms that Phone B has
received the 200 OK response from Phone C.
The ACK might contain a message body with the final session description to be
used by Phone C. If the message body of the ACK is empty, Phone C uses the
session description in the INVITE request.
Phone B sends a midcall INVITE to Phone C 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.
Phone A sends a SIP 200 OK response to Phone B.
Phone B sends a SIP ACK to Phone C. The ACK confirms that Phone B has
received the 200 OK response from Phone C.
Phone B sends a REFER message to Phone A. The message contains the
following information:
• Refer-To: C
Replaces: B
•
Referred-By: B
•
The message indicates that the user (recipient) should contact a third party for use
in transferring parties.
Phone A sends a SIP 202 ACCEPTED message to Phone B. The confirms that the
REFER message has been received.
Phone A sends a NOTIFY message to Phone B. This message notifies B that the
REFER processing has started.
Appendix B SIP Call Flows
OL-7890-01