Is your Audiocode's ECM support badly broken? Spandsp's T.38 terminal and gateway engines have been tested with numerous systems, and ECM is normally OK unless the other system does something stupid - which unfortunately a number of T.38 implementations still do.
It might be, although it doesn't work with it turned off either. These gateways have been in use for about a year, going through Asterisk to a SIP trunking provider that uses Metaswitch, and for the most part, they work well. It's when I terminate the call to Freeswitch and SpanDSP that problems arise, but only with this device. I'm waiting to get one back from a customer, then I can do a proper A-B comparison using wireshark.
When ITSPs recommend turning off ECM, its just an admission of failure. They are accepting that the networks are so poor they are unlikely to get a 100% correct result, even after the permitted number of retries. In other words, they are accepting that FAXes with numerous errors, like we had in the bad old days before ECM, are the bold new standard for service quality in the internet era. Wonderful, isn't it.
ECM is there for a reason. Our upstream carriers are great (otherwise we would choose someone else), but most of our customers are connecting over unmanaged DSL links, so there are going to be a few glitches. Testing with my own fax machine, ECM off usually ends up with a few bad lines, but ECM on gives me clean faxes every time. A lot can be said for machines too. 99% of the "half my faxes fail" calls come from customers with HP all-in-one machines. I have an old brother fax machine that someone was throwing away. If I set it up at the same customer site, they can fax without problems. That usually convinces them to buy a new fax machine.