Just a heads up guys, I have a customer who has been complaining of fax failures since they went over to Business SIP. The fax service is delivered over a separate ATA but the customer was getting constant fax failures.
It turns out that the customer who is a medical practice, faxes mainly to a medical imaging company who uses a Vocus fax stream type service and due to a handshaking issue with that service which only applies to faxes from a SIP service, it switches to a wrong mode and the fax fails. I am not 100% sure of the technicalities but Telstra is liasing with Vocus to resolve the issue. So if you have customers complaining of faxes failing, this could be the issue!