Yealink SIP-T54S Administrator's Manual page 1098

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
User B accepts the call from User C.
4.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
F10. INVITE A
F11. 180 Ringing
F13. INVITE B ( sendonly )
F316 200 OK
F17. ACK
F19. 200 OK
F22. ACK
Step
F1 INVITE—User A to Proxy Server
1072
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
2-way RTP channel established
F14. INVITE B ( sendonly )
F15. 200 OK
F18. ACK
No RTP Packets being sent
Action
User B
F9. INVITE A
F12. 180 Ringing
F20. 200 OK
F21. 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 A is identified as the call

User C