Yealink SIP-T48G Administrator's Manual page 592

Administrator's Guide for SIP-T2_Series_T4_Series IP Phones
Step
INVITE—User A to Proxy
F1
Server
INVITE—Proxy Server to User
F2
B
180 Ringing—User B to Proxy
F3
Server
180 Ringing—Proxy Server to
F4
User A
200 OK—User B to Proxy
F5
Server
578
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 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 200 OK response to
the proxy server. The 200 OK response
notifies the proxy server that the
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.