Cisco SIP IP Phone Administrator's Manual page 146

Call Flow Scenarios for Successful Calls
Step Action
INVITE—Cisco SIP IP phone A
1.
to SIP proxy server
INVITE—SIP proxy server to
2.
SIP redirect server
302 Moved Temporarily—SIP
3.
redirect server to SIP proxy
server
INVITE—SIP proxy server to
4.
Cisco SIP IP phone C
180 Ringing—Cisco SIP IP
5.
phone C to SIP proxy server
200 OK—Cisco SIP IP phone C
6.
to SIP proxy server
200 OK—SIP proxy server to
7.
Cisco SIP IP phone A
ACK—Cisco SIP IP phone A to
8.
SIP proxy server
ACK—SIP proxy server to Cisco
9.
SIP IP phone C
Cisco SIP IP Phone Administrator Guide
B-36
Description
Cisco SIP IP phone A sends a SIP INVITE request to the SIP proxy server. The
INVITE 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:555-0002@companyb.com; user=phone." The "user=phone" parameter
distinquishes that the Request-URI address is a telephone number rather than
a username.
Cisco SIP IP 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.
•
SIP proxy server sends the SIP INVITE request to the SIP redirect server.
SIP redirect server sends a SIP 302 Moved temporarily message to the SIP proxy
server. The message indicates that User B is not available at SIP phone B and
includes instructions to locate User B at Cisco SIP IP phone C.
SIP proxy server sends a SIP INVITE request to Cisco SIP IP phone C. The
INVITE request is an invitation to User C to participate in a call session.
Cisco SIP IP phone C sends a SIP 180 Ringing response to the SIP proxy server.
Cisco SIP IP phone C sends a SIP 200 OK response to the SIP proxy server.
SIP proxy server forwards the SIP 200 OK response to Cisco SIP IP phone A.
Cisco SIP IP phone A sends a SIP ACK to the SIP proxy server. The ACK
confirms that the SIP proxy server has received the 200 OK response from
Cisco SIP IP phone C.
SIP proxy server forwards the SIP ACK to the Cisco SIP IP phone C. The ACK
confirms that Cisco SIP IP phone A has received the 200 OK response from
Cisco SIP IP phone C.
Appendix B SIP Call Flows