by Liptonbrisk » 08/27/2020
Although faxing isn't officially supported, FPL does work with T.38 fax protocol.
For Grandstream ATAs, use these settings found in the FXS Port tab:
1) Re-INVITE After Fax Tone Detected: Enabled
2) Fax Mode: T.38
3) Fax Tone Detection Mode: Caller or Callee
4) Use First Matching Vocoder in 200OK SDP: Yes
5) Preferred Vocoder: choice 1: PCMU
6) Disable Line Echo Canceller (LEC): Yes
7) Disable Call-Waiting: Yes
8) Disable Call-Waiting Tone: Yes (not sure if this needs to be disabled if #7 is disabled anyway, but the tone here is definitely bad for faxing)
a) I'm not sure if you also need to disable Call-Waiting Caller ID with #7 and #8 disabled, but disabling it wouldn't hurt for faxing. I don't have your ATA to test. Disable just in case.
9) On your fax machine or printer, set fax transmit or baud rate to 14,400. If that doesn't work, try 9600. If 14,400 works, you can try faster speeds.
I have faxed at 33600 successfully using FPL with an OBi202.
10) If you have problems, try changing Jitter Buffer Type to Fixed from Adaptive in your ATA.