Yealink SIP-T48G Administrator's Manual page 589

The call flow scenario is as follows:
1. User A calls User B.
2. User B does not answer the call.
3. User A hangs up.
The call cannot be set up successfully.
User A
F1. INVITE B
F4. 180 Ringing
F5. CANCEL
F8. 200 OK
Step
INVITE—User A to Proxy
F1
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:
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
Appendix
User B
575