Cisco 7960G Administration Manual page 153

Appendix B SIP Call Flows
Called Number Is Busy
Figure B-21
the phone and is unable or unwilling to take another call.
Figure B-21 Called Number Is Busy
Phone User A
Step Action
INVITE—Phone A to Phone B
1.
486 Busy Here—Phone B to
2.
Phone A
3. ACK—Phone A to Phone B
OL-7890-01
illustrates an unsuccessful call in which User A initiates a call to User B, but User B is on
SIP IP
IP
1. INVITE B
2. 486 Busy Here
3. ACK
Description
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:555-0199@companyb.com; 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.
•
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.
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.
Cisco Unified IP Phone 7960G and 7940G Administration Guide for Release 8.0 (SIP)
Call Flow Scenarios for Failed Calls
IP Network
SIP IP
Phone User B
IP
B-53