Yealink SIP-T54S Administrator's Manual page 1106

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
Call is established between User B and User C.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
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
Step
F1 INVITE—User A to Proxy Server
1080
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
2-way RTP channel established
F10. INVITE B （sendonly）
F11. 200 OK
F14. ACK
2-way RTP channel established
F25. REFER
F26. 202 Accepted
F32. BYE
F33. 200 OK
Action
User B
F16. INVITE C
F17. 180 Ringing
F19. 200 OK
F22. ACK
2-way RTP channel established
Description
User A sends an 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:
The IP address of User B is inserted in

the Request-URI field.
User C