Yealink SIP-T2XP Administrator's Manual page 551

Step Action
INVITE—User A to Proxy
F1
Server
INVITE—Proxy Server to User
F2
B
100 Trying—User B to Proxy
F3
Server
100 Trying—Proxy Server to
F4
User A
180 Ringing—User B to Proxy
F5
Server
180 Ringing—Proxy Server to
F6
User A
Description
User A sends a SIP 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 100 Trying response
to the proxy server. The 100 Trying
response indicates that the INVITE
request has been received by User B.
The proxy server forwards the SIP 100
Trying to User A to indicate that the
INVITE request has been received by
User B.
User B sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the User B 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.
Appendix
533