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Appendix
1077
Step Action Description

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 specified.
F2 INVITE—Proxy Server to User B
The proxy server maps the SIP URI in the
To field to User B. The proxy server sends
the INVITE message to User B.
F3
180 Ringing—User B to Proxy
server
User B sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
F4
180 Ringing—Proxy Server to
User A
The proxy server forwards the 180 Ringing
response to User A. User A hears the ring-
back tone indicating that User B is being
alerted.
F5 200 OK—User B to Proxy Server
User B sends a SIP 200 OK response to the
proxy server. The 200 OK response
notifies User A that the connection has
been made.
F6 200 OK—Proxy Server to User A
The proxy server forwards the 200 OK
message to User A. The 200 OK response
notifies User A that the connection has
been made.
F7 ACK—User A to Proxy Server
User A sends a SIP ACK to the proxy
server, The ACK confirms that User A has
received the 200 OK response. The call
session is now active.
F8 ACK—Proxy Server to User B
The proxy server sends the SIP ACK to
User B. The ACK confirms that the proxy