Yealink SIP-T54S Administrator's Manual page 1093

The call cannot be set up successfully.
User A
F1. INVITE B
F4. 180 Ringing
F5. CANCEL
F8. 200 OK
Step
F1 INVITE—User A to Proxy Server
F2 INVITE—Proxy Server to User B
180 Ringing—User B to Proxy
F3
Server
Proxy Server
Action
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 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
F2. INVITE B
F3. 180 Ringing
F6. CANCEL
F7. 200 OK
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.
Appendix
User B
1067