Fax not recognizing carrier in T38


I have the following setup:
fax <–> ata <–> asterisk <----> t38 provider

the ata is either a grandstream ht503 or a mediatrix 2102

The call is setup correctly, reinvite are sent to switch to t38. Then, the carrier / fax tone is heard on the fax, but the fax doesnt recognize the transmission speed, and hangs up.

ECM is disabled, speed has been tested between 2400 & 9600, no luck. If i direct the fax call to an asterisk extension with a ReceiveFax, it works everytime. I’ve contacted the voip provider, and they cant help as it is my fax hanging up.

Any suggestion as what to tune to get this working ?

Many thanks,