
B-54
Cisco SIP IP Phone Administrator Guide
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Figure B-19 Cisco SIP IP Phone-to-Cisco SIP IP Phone—Authentication Error
Call from a Cisco SIP IP Phone to a Gateway Acting as a Backup Proxy
Figure B-20
illustrates a successful call from a Cisco SIP IP phone to a gateway acting as a backup
proxy.
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.
407 Authentication Error—SIP
proxy server to Cisco SIP IP
phone A
SIP proxy server sends a SIP 407 Authentication Error response to Cisco SIP IP
phone A.
3.
ACK—Cisco SIP IP phone A to
SIP proxy server
Cisco SIP IP phone A sends a SIP ACK to the SIP proxy server acknowledging
the 407 error message.
4.
Resend INVITE—Cisco SIP IP
phone A to SIP proxy server
Cisco SIP IP phone A resends a SIP INVITE to the SIP proxy server with
authentication credentials.
Proxy
Server
IP
IP
2. 407 Authentication Error
4. Resend INVITE B
3. ACK
1. INVITE B
IP Network
SIP IP
Phone User A
SIP IP
Phone User B
41477