Yealink SIP-T46G Administrator's Manual page 387

The call flow scenario is as follows:
1.
2.
User A
VoIPon www.voipon.co.uk sales@voipon.co.uk Tel: +44 (0)1245 808195 Fax: +44 (0)1245 808299
User A calls User B.
User B is busy on the IP phone and unable or unwilling to take another call.
The call cannot be set up successfully.
F1. INVITE B
F4. 100 Trying
F6. 486 Busy Here
F7. ACK
Proxy Server
F2. INVITE B
F3. 100 Trying
F5. 486 Busy Here
F8. ACK
Appendix
User B
375