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Cisco SIP IP Phone 7960 Administrator Guide
78-10497-02
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
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 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.
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.
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