Yealink SIP-T48G Administrator's Manual page 618

Administrator's Guide for SIP-T2_Series_T4_Series IP Phones
which User A mixes two RTP channels and therefore establishes a conference between
User B and User C. In this call flow scenario, the end users are User A, User B, and User
C. They are all using Yealink SIP IP phones, which are connected via an IP network.
The call flow scenario is as follows:
1. User A calls User B.
2. User B answers the call.
3. User A places User B on hold.
4. User A calls User C.
5. User C answers the call.
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
604
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