Yealink SIP-T48G Administrator's Manual page 304

Microsoft Lync Edition Administrator's Guide for SIP-T2_Series and T4_Series IP Phones
Step
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
292
Action
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 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.
Description
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.