Cisco 7960 Administrator's Manual page 175

Appendix B SIP Call Flows
Step Action
1
INVITE—Cisco SIP IP phone A
to Cisco SIP IP phone B
2
486 Busy Here—Cisco SIP IP
phone B to Cisco SIP IP phone A
3
ACK—Cisco SIP IP phone A to
Cisco SIP IP phone B
78-10497-02
Description
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 email 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:555-0002@companyb.com;
user=phone." The "user=phone" parameter
distinquishes that the Request-URI address is a
telephone number rather than a user name.
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.
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.
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.
Cisco SIP IP Phone 7960 Administrator Guide
Call Flow Scenarios for Failed Calls
B-67