Yealink SIP-T2xP Administrator's Manual page 485

Step Action
INVITE—User A to Proxy
F1
Server
INVITE—Proxy Server to User
F2
B
302 Move Temporarily—User
F3
B to Proxy Server
F4 ACK—Proxy Server to User B
302 Move Temporarily—Proxy
F5
Server to User A
F6 ACK—User A to Proxy Server
Description
User A sends an 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 the 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.
The proxy server maps the SIP URI in the
To field to User B. The proxy server
sends the INVITE message to User B.
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.
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.
The proxy server forwards the 302
Moved Temporarily message to User A.
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.
Appendix
469