Yealink SIP-T54S Administrator's Manual page 1107

Step Action
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
F6 200 OK—Proxy Server to User A
F7 ACK—User A to Proxy Server
F8 ACK—Proxy Server to User B
Description
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 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.
User A sends a SIP ACK to the proxy
server, The ACK confirms that User A has
received the 200 OK response. The call
session is now active.
The proxy server sends the SIP ACK to
User B. The ACK confirms that the proxy
server has received the 200 OK response.
The call session is now active.
Appendix
1081