My question would be why does it matter if you get a busy or ringback? To an incoming fax machine the tone it actually receives is irrelevant since the only thing it really wants is to receive a fax tone to start transmission, if it doesn't connect after a short time (like 45 seconds or so), it will disconnect, then most will wait a while, and try again. The only real difference might be how it reports it if it does reports, it might say "No Answer" instead of "Busy" which to a fax machine doesn't matter, it just means the fax didn't complete.
