There’s no one tried & true way of sending faxes & low speed data but there are some basic techniques which may help in some situations on most generic ATAs:
- disable silence suppression on the ATA
- disable fax detection on the ATA
- disable T.38 along the entire call path (if possible)
- ensure only codec for negotiation is aLaw for entire call path
- start out by lowering the fax speed down to 9600, get this working & then try 14400, then 28800 etc…
Normally this takes it from about a 20% success rate on fax calls to around 96% i.e. your mileage may vary