Yealink SIP-T2xP Administrator's Manual page 484

Administrator's Guide for SIP-T2xP and SIP-T19P IP Phones
4. User C answers the call.
Call is established between User A and User C.
User A
F1. INVITE B
F5. 302 Move Temporarily
F6. ACK
F7. INVITE C
F10. 180 Ringing
F12. 200 OK
F13. ACK
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Proxy Server
F2. INVITE B
F3. 302 Move Temporarily
F4. ACK
F8. INVITE C
F9. 180 Ringing
F11. 200 OK
F14. ACK
2-way RTP channel established
User B User C