Yealink SIP-T2xP Administrator's Manual page 470

Administrator's Guide for SIP-T2xP and SIP-T19P IP Phones
The call flow scenario is as follows:
1. User A calls User B.
2. User B answers the call.
3. User C calls User B.
4. User B accepts the call from User C.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
2-way RTP channel established
F10. INVITE A
F11. 180 Ringing
F13. INVITE B ( sendonly )
F316 200 OK
F17. ACK
No RTP Packets being sent
F19. 200 OK
F22. ACK
454
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
F14. INVITE B ( sendonly )
F15. 200 OK
F18. ACK
2-way RTP channel established
User B
F9. INVITE A
F12. 180 Ringing
F20. 200 OK
F21. ACK
User C