Yealink SIP-T38G Administrator's Manual page 368

Step Action
F7 ACK—User A to Proxy Server
F8 ACK—Proxy Server to User B
INVITE—User C to Proxy
F9
Server
INVITE—Proxy Server to User
F10
A
180 Ringing—User A to Proxy
F11
Server
180 Ringing—Proxy Server to
F12
User C
Description
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.
User C sends a SIP INVITE message to
the proxy server. The INVITE request is
an invitation to User A to participate in
a call session.
In the INVITE request:
The IP address of User A is inserted

in the Request-URI field.
User C 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 C is

ready to receive is specified.
The port on which User A is

prepared to receive the RTP data is
specified.
The proxy server maps the SIP URI in the
To field to User A. The proxy server
sends the INVITE message to User A.
User A 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 C. User C
hears the ring-back tone indicating that
User A is being alerted.
Appendix
355