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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
Call Transfer Without Consultation Using Failover
Figure B-8
illustrates a successful call between Cisco SIP IP phones in which two parties are in a call
and then one of the participants transfers the call to a third party without first contacting the third party.
This is called a blind or unattended transfer. 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 using 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 transfers the call to User C.
9.
REFER—Phone B to Phone A
Phone B sends a REFER message to Phone A. The message contains the
following information:
•
Refer-To: C
•
Referred-By: B
The message indicates that Phone A should send an INVITE request to
Phone C.
10.
202 ACCEPTED—Phone A to
Phone B
Phone A sends a SIP 202 ACCEPTED message to Phone B. The message
confirms that the REFER message has been received.
11.
NOTIFY (Event:Refer;
Subscription-State: Active)
Phone A sends a NOTIFY message to Phone B. This message notifies B that
the REFER processing has started.
12.
BYE—Phone B to Phone A
Phone B sends a BYE message to Phone A. The message indicates that
Phone B will disconnect from the call.
13.
200 OK—Phone A to Phone B
Phone A sends a SIP 200 OK response to Phone B. The response notifies
Phone B that the BYE message was received.
14.
INVITE—Phone A to Phone C
Because of the REFER message from Phone B, Phone A sends a SIP INVITE
request to Phone C. The request is an invitation to User C to participate in a
call session. The request contains the following information:
•
Referred-By: B
The message indicates that the INVITE was referred by Phone B.
15.
100 Trying—Phone C to Phone A
Phone C sends a SIP 100 Trying response to Phone A. The response indicates
that the INVITE request has been received.
16.
180 Ringing—Phone C to Phone A
Phone C sends a SIP 180 Ringing response to Phone A.
17.
200 OK—Phone C to Phone A
Phone C sends a SIP 200 OK response to Phone A. The response notifies
Phone A that the connection has been made.
18.
ACK—Phone A to Phone C
Phone A sends a SIP ACK to Phone C. The ACK confirms that Phone A has
received the 200 OK response from Phone C.
19.
NOTIFY—Phone A to Phone B
Phone A sends a NOTIFY message to Phone B. The message notifies
Phone C of the REFER event.
20.
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 NOTIFY message was received.
A two-way RTP channel is established between Phone A and Phone C.
Step
Action
Description
