Yealink SIP-T2XP Administrator's Manual page 454

Administrator's Guide for SIP-T2xP IP Phones
Step
F1 INVITE—User A to Proxy Server
F2 INVITE—Proxy Server to User B
180 Ringing—User B to Proxy
F3
Server
180 Ringing—Proxy Server to
F4
User A
F5 200 OK—User B to Proxy Server
200 OK—Proxy Server to User
F6
A
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Action
User A sends the 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. Proxy server forwards
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 User A that the connection has
been made.
The proxy server forwards the 200 OK
message to User A. The 200 OK response
notifies User A that the connection has
been made.
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.