Yealink SIP-T54S Administrator's Manual page 1113

The call flow scenario is as follows:
User B enables busy call forward, and the destination number is User C.
1.
User A calls User B.
2.
User B is busy.
3.
User B forwards the incoming call to User C.
4.
User C answers the call.
5.
Call is established between User A and User C.
User A
F1. INVITE B
F4. 180 Ringing
F7. 302 Move Temporarily
F8. ACK
F9. INVITE C
F12. 180 Ringing
F14. 200 OK
F15. ACK
Step
F1 INVITE—User A to Proxy Server
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 302 Move Temporarily
F6. ACK
F10. INVITE C
F11. 180 Ringing
F13. 200 OK
F16. ACK
2-way RTP channel established
Action
User A sends the 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:



User B
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
Appendix
User C
1087