Yealink SIP-T2 Series Administrator's Manual page 520

Administrator's Guide for SIP-T23 & T23P & T23G IP Phones
5. User A transfers the call to User C.
Call is established between User B and User C.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
2-way RTP channel established
F9. INVITE B （sendonly）
F12. 200 OK
F13. ACK
F15. INVITE C
F18. 180 Ringing
F20. 200 OK
F21. ACK
F23. REFER
F24. 202 Accepted
F31. BYE
F34. 200 OK
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Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
F10. INVITE B （sendonly）
F11. 200 OK
F14. ACK
2-way RTP channel established
F25. REFER
F26. 202 Accepted
F32. BYE
F33. 200 OK
2-way RTP channel established
User B
F16. INVITE C
F17. 180 Ringing
F19. 200 OK
F22. ACK
User C