Yealink SIP-T54S Administrator's Manual page 1102

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
The call flow scenario is as follows:
User A calls User B.
1.
User B answers the call.
2.
User B transfers the call to User C.
3.
User C answers the call.
4.
Call is established between User A and User C.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
F11. REFER
F12. 202 Accepted
F18. BYE
F19. 200 OK
F21. INVITE C
F24. 180 Ringing
F26. 200 OK
F27. ACK
Step
F1 INVITE—User A to Proxy Server
1076
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
2-way RTP channel established
F9. REFER
F10. 202 Accepted
F17. BYE
F20. 200 OK
2-way RTP channel established
Action
User B
F22. INVITE C
F23. 180 Ringing
F25. 200 OK
F28. ACK
Description
User A sends an INVITE message to the
proxy server. The INVITE request is an
invitation to User B to participate in a call
session.
In the INVITE request:
User C