Yealink SIP-T48G Administrator's Manual page 612

Administrator's Guide for SIP-T2_Series_T4_Series IP Phones
C. They are all using Yealink SIP IP phones, which are connected via an IP network.
The call flow scenario is as follows:
1. User B enables busy call forward, and the destination number is User C.
2. User A calls User B.
3. User B is busy.
4. User B forwards the incoming call to User C.
5. User C answers the call.
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
598
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
User B User C