Call is established between User A and User C.
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.