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Appendix
415
The call flow scenario is as follows:
1. User A calls User B.
2. User B is busy on the IP phone and unable or unwilling to take another call.
The call cannot be set up successfully.
User A Proxy Server User B
F1. INVITE B
F2. INVITE B
F3. 100 Trying
F4. 100 Trying
F5. 486 Busy Here
F6. 486 Busy Here
F7. ACK
F8. ACK
Step Action Description
F1
INVITE—User A to Proxy
Server
User A sends the INVITE message to a
proxy server. The INVITE request is an
invitation to User B to participate in a
call session.
In the INVITE request:

The IP address of User B is inserted
in the Request-URI field.

User 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.

The port on which User B is
prepared to receive the RTP data is