Yealink SIP-T2XP Administrator's Manual page 459

Step Action
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
200 OK—Proxy Server to User
F6
A
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 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 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 proxy server 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.
Appendix
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