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Cisco SIP IP Phone Administrator Guide
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Cisco SIP IP Phone-to-Cisco SIP IP Phone—Authentication Error
Figure B-19
illustrates an unsuccessful call in which User A initiates a call to User B but is prompted
for authentication credentials by the proxy server. User A’s SIP IP phone then reinitiates the call with an
SIP INVITE request that includes it’s authentication credentials.
Step
Action
Description
1.
INVITE—Cisco SIP IP phone A
to Cisco SIP IP phone B
Cisco SIP IP phone A 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.
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.
180 Ringing—Cisco SIP IP
phone B to Cisco SIP IP phone A
Cisco SIP IP phone B sends a SIP 180 Ringing response to Cisco SIP IP phone A.
3.
CANCEL (Ring
Timeout)—Cisco SIP IP phone
A to Cisco SIP IP phone B
Cisco SIP IP phone A sends a CANCEL request to Cisco SIP IP phone B to
cancel the invitation.
4.
200 OK—Cisco SIP IP phone B
to Cisco SIP IP phone A
Cisco SIP IP phone B sends a SIP 200 OK response to Cisco SIP IP phone A. The
response confirms receipt of the cancellation request.
