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Cisco SIP IP Phone Administrator Guide
Appendix B SIP Call Flows
Call Flow Scenarios for Successful Calls
Step
Action
Description
1.
INVITE—Cisco SIP IP phone A
to SIP proxy server
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:[email protected]; 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.
2.
INVITE—SIP proxy server to
SIP redirect server
SIP proxy server sends the SIP INVITE request to the SIP redirect server.
3.
300 Multiple Choices—SIP
redirect server to SIP proxy
server
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 SIP phone B or
Cisco SIP IP phone C.
4.
INVITE—SIP proxy server to
Cisco SIP IP phone B
SIP proxy server sends a SIP INVITE request to Cisco SIP IP phone B. The
INVITE request is an invitation to User B to participate in a call session.
5.
486 Busy Here—Cisco SIP IP
phone B to SIP proxy server
Cisco SIP IP phone B sends a 486 Busy here message to the SIP proxy server. The
message indicates that Cisco SIP IP phone B is in use and the user is not willing
or able to take additional calls.
6.
ACK—SIP proxy server to
Cisco SIP IP phone B
SIP proxy server forwards the SIP ACK to the Cisco SIP IP phone B. The ACK
confirms that the SIP proxy server has received the 486 Busy here response from
Cisco SIP IP phone B.
7.
INVITE—SIP proxy server to
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.
8.
180 Ringing—Cisco SIP IP
phone C to SIP proxy server
Cisco SIP IP phone C sends a SIP 180 Ringing response to the SIP proxy server
9.
200 OK—Cisco SIP IP phone C
to SIP proxy server
Cisco SIP IP phone C sends a SIP 200 OK response to the SIP proxy server.
10.
200 OK—SIP proxy server to
Cisco SIP IP phone A
SIP proxy server forwards the SIP 200 OK response to Cisco SIP IP phone A.