Yealink SIP-T21 E2

Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
1076
The call flow scenario is as follows:
1. User A calls User B.
2. User B answers the call.
3. User B transfers the call to User C.
4. User C answers the call.
Call is established between User A and User C.
User A Proxy Server User B
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
2-way RTP channel established
F23. 180 Ringing
F11. REFER
F12. 202 Accepted
F26. 200 OK
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
F22. INVITE C
F19. 200 OK
F27. ACK
F18. BYE
2-way RTP channel established
F9. REFER
F10. 202 Accepted
F17. BYE
F20. 200 OK
F21. INVITE C
F24. 180 Ringing
F25. 200 OK
F28. ACK
User C
Step
Action
Description
F1
INVITE—User A to Proxy Server
User A sends an INVITE message to the
proxy server. The INVITE request is an
invitation to User B to participate in a call
session.
In the INVITE request: