I received a good tip here, so it should work:
Grandstream ATA ( HT8XX
Getting Fax machines to function correctly on a Grandstream Analog Gateway.
You need to provision it for “FAX MODE” = Pass-Through
Symmetric RTP
Use First Matching Vocoder in 200OK SDP = PCMU
Below is screen shot of GDMS settings.
When you set that correctly, it conflicts with this:
“Call Feature”
You must disable them in GDMS for some reason, but, by default they are on in the device and work fine.