I have a system on 9.1.4 with a phone 2 and sip trunks.

If the fax dials out but receives a busy tone, the line does not clear down and the fax continues to send the fax.

SIP is with Gamma.

I am using Fax Transport Support of G.711

Sending from a fax machine? They don't actually recognize busy from no answer from wrong number. All they recognize is successful connect. So if it doesn't connect, it will try to negotiate until it times out, that's when it will disconnect, go into a waiting mode, and then retry later.