Yealink W60P Administrator's Manual page 541

User A mixes the RTP channels and establishes a conference between User B and User C.
6.
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
Both calls are active, come into three-party conference
Step
F1 INVITE—User A to Proxy Server
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
Action
User A sends the INVITE message to a
proxy server. The INVITE request is an
invitation to User B to participate in a call
session.
In the INVITE request:




User B
Description
The IP address of User B is inserted
in the Request-URI field.
User A is identified as the call
session initiator in the From field.
A unique numeric identifier is
assigned to the call and is inserted in
the Call-ID field.
The transaction number within a
Appendix
User C
521