Yealink SIP-T2 Series Administrator's Manual page 516

Administrator's Guide for SIP-T23 & T23P & T23G IP Phones
4. User C answers the call.
Call is established between User A and User C.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
2-way RTP channel established
F11. REFER
F12. 202 Accepted
F18. BYE
F19. 200 OK
F21. INVITE C
F24. 180 Ringing
F26. 200 OK
F27. ACK
504
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
F9. REFER
F10. 202 Accepted
F17. BYE
F20. 200 OK
2-way RTP channel established
User B
F22. INVITE C
F23. 180 Ringing
F25. 200 OK
F28. ACK
User C