B-52
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—Called User Does Not Answer
Figure B-18
illustrates an unsuccessful call in which User A initiates a call to User B but User B does
not answer.
Figure B-18 Cisco SIP IP Phone-to-Cisco SIP IP Phone—Called User Does Not Answer
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.
486 Busy Here—Cisco SIP IP
phone B to Cisco SIP IP phone A
Cisco SIP IP phone B sends a 486 Busy here message to the Cisco SIP IP phone A.
The message indicates that Cisco SIP IP phone B is in use and the user is not
willing or able to take additional calls.
3.
ACK—Cisco SIP IP phone A to
Cisco SIP IP phone B
Cisco SIP IP phone A sends a SIP ACK to the Cisco SIP IP phone B. The ACK
confirms that Cisco SIP IP phone A has received the 486 Busy Here response from
Cisco SIP IP phone B.
IP
IP
2. 180 Ringing
4. 200 OK
3. CANCEL
1. INVITE B
IP Network
SIP IP
Phone User A
SIP IP
Phone User B
41476
