Yealink SIP-T2XP Administrator's Manual page 452

Administrator's Guide for SIP-T2xP and SIP-T19P IP Phones
The call flow scenario is as follows:
1. User A calls User B.
2. User B does not answer the call.
3. User A hangs up.
The call cannot be set up successfully.
User A
F1. INVITE B
F4. 180 Ringing
F5. CANCEL
F8. 200 OK
436
Proxy Server
F2. INVITE B
F3. 180 Ringing
F6. CANCEL
F7. 200 OK
User B