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Appendix
557
Step Action Description

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
302 Move Temporarily—User
B to Proxy Server
User B sends a SIP 302 Moved
Temporarily message to the proxy
server. The message indicates that User
B is not available at SIP phone B. User B
rewrites the contact-URI.
F4 ACK—Proxy Server to User B
The proxy server sends a SIP ACK to
User B, the ACK message notifies User B
that the proxy server has received the
302 Move Temporarily message.
F5
302 Move Temporarily—Proxy
Server to User A The proxy server forwards the 302
Moved Temporarily message to User A.
F6 ACK—User A to Proxy Server
User A sends a SIP ACK to the proxy
server. The ACK message notifies the
proxy server that User A has received
the 302 Move Temporarily message.
F7
INVITE—User A to Proxy
Server
User A sends a SIP INVITE request to the
proxy server. In the INVITE request, a
unique Call-ID is generated and the
Contact-URI field indicates that User A
requested the call.
F8
INVITE—Proxy Server to User
C
The proxy server maps the SIP URI in the
To field to User C. The proxy server
sends the SIP INVITE request to User C.
F9
180 Ringing—User C to Proxy
Server
User C sends a SIP 180 Ringing
response to the proxy server. The 180
Ringing response indicates that the user
is being alerted.
F10
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