B-37
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
Step
Action
Description
1.
INVITE—Phone A to SIP proxy
server
Phone A sends a SIP INVITE request to the SIP proxy server. The request is an
invitation to User B to participate in a call session. In the INVITE request:
•
The phone number of User B is inserted in the Request-URI field in the form
of a SIP URL. The SIP URL identifies the address of User B and takes a form
similar to an e-mail address (
user
@
host
, where
user
is the telephone number
and
host
is either a domain name or a numeric network address). For example,
the Request-URI field in the INVITE request to User B appears as “INVITE
sip:[email protected]; user=phone.” The “user=phone” parameter
distinquishes that the Request-URI address is a telephone number rather than
a username.
•
Phone A is identified as the call session initiator in the From field.
•
A unique numeric identifier is assigned to the call and is inserted in the
Call-ID field.
•
The transaction number within a single call leg is identified in the CSeq field.
•
The media capability User A is ready to receive is specified.
2.
INVITE—SIP proxy server to
SIP redirect server
The SIP proxy server sends the SIP INVITE request to the SIP redirect server.
3.
300 Multiple Choices—SIP
redirect server to SIP proxy
server
The SIP redirect server sends a SIP 300 Multiple choices message to the SIP proxy
server. The message indicates that User B can be reached either at Phone B or
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 B
The SIP proxy server sends a SIP INVITE request to Phone B. The request is an
invitation to User B to participate in a call session.
6.
180 Ringing—Phone B to SIP
proxy server
Phone B sends a SIP 180 Ringing response to the SIP proxy server.
7.
180 Ringing—SIP proxy server
to Phone A
The SIP proxy server sends a SIP 180 Ringing response to Phone A.
The timeout expires before the phone is answered.
8.
CANCEL (Ring Timeout)—SIP
proxy server to Phone B
The SIP proxy server sends a CANCEL request to Phone B to cancel the
invitation.
9.
200 OK—Phone B to SIP proxy
server
Phone B sends a SIP 200 OK response to the SIP proxy server. The response
confirms receipt of the cancellation request.
10.
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.
11.
180 Ringing—Phone C to SIP
proxy server
Phone C sends a SIP 180 Ringing response to the SIP proxy server.
12.
200 OK—Phone C to SIP proxy
server
Phone C sends a SIP 200 OK response to the SIP proxy server.
13.
200 OK—SIP proxy server to
Phone A
The SIP proxy server forwards the SIP 200 OK response to Phone A.