Yealink SIP-T48G Administrator's Manual page 279

The call flow scenario is as follows:
1. User A calls User B.
2. User B answers the call.
3. User B hangs up.
User A
F1. INVITE B
F4. 100 Trying
F6. 180 Ringing
F8. 200 OK
F9. ACK
F12. BYE
F13. 200 OK
Step
INVITE—User A to Proxy
F1
Server
Proxy Server
2-way RTP channel established
Action
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:



F2. INVITE B
F3. 100 Trying
F5. 180 Ringing
F7. 200 OK
F10. ACK
F11. BYE
F14. 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
Appendix
User B
267