Yealink SIP-T48G Administrator's Manual page 616

Administrator's Guide for SIP-T2_Series_T4_Series IP Phones
Step
INVITE—Proxy Server to User
F2
B
180 Ringing—User B to Proxy
F3
Server
180 Ringing—Proxy Server to
F4
User A
302 Move Temporarily—User
F5
B to Proxy Server
F6 ACK—Proxy Server to User B
302 Move Temporarily—Proxy
F7
Server to User A
602
Action
call session.
In the INVITE request:
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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 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
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.
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
ACK message.
The proxy server forwards the 302
Moved Temporarily message to User A.
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.