Yealink SIP-T2XP Administrator's Manual page 482

Administrator's Guide for SIP-T2xP and SIP-T19P IP Phones
6. User A mixes the RTP channels and establishes a conference between User B and
User C.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
Session1 established between User A and User B is active
F9. INVITE(sendonly)
Initiate
three party
conference
F12. 200 OK
F13. ACK
Session 1 established between User A and User B is hold
F15. INVITE C
F18. 180 Ringing
F20. 200 OK
F21. ACK
466
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
F10. INVITE (sendonly)
F11. 200 OK
F14. ACK
F16. INVITE C
F17. 180 Ringing
F19. 200 OK
F22. ACK
Both calls are active, come into three-party conference
User B User C