B-53
Cisco Unified IP Phone 7960G and 7940G Administration Guide for Release 8.0 (SIP)
OL-7890-01
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Called Number Is Busy
Figure B-21
illustrates an unsuccessful call in which User A initiates a call to User B, but User B is on
the phone and is unable or unwilling to take another call.
Figure B-21 Called Number Is Busy
Step
Action
Description
1.
INVITE—Phone A to Phone B
Phone A sends a SIP INVITE request to Phone B. The 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 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 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.
•
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 A is ready to receive is specified.
2.
486 Busy Here—Phone B to
Phone A
Phone B sends a 486 Busy Here message to the Phone A. The message indicates
that Phone B is in use and the user is not willing or able to take additional calls.
3.
ACK—Phone A to Phone B
Phone A sends a SIP ACK to the Phone B. The ACK confirms that Phone A has
received the 486 Busy Here response from Phone B.
IP
IP
2. 486 Busy Here
3. ACK
1. INVITE B
IP Network
SIP IP
Phone User A
SIP IP
Phone User B
41475
