DTMF (Dual-Tone Multi-Frequency) is, essentially, touch tones or sounds heard when a number is pressed on a dial pad.
Changing DTMFMethod to Inband addresses DTMF talkoff (unwanted touch tone sounds or beeps), which is triggered usually by high-pitched voices and by high-pitched sounds. It will, otherwise, not fix choppy audio and would have been a coincidental solution, unless some bug in your device is involved. I've had DTMFMethod set to Auto for approximately 10 years on OBi202 ATA without choppy audio.
The reason why reducing volume may help for DTMF talkoff is that it will lower the chance of a high-pitched noise triggering an unwanted touch tone sound or beep.
It also helps not to use hands-free calling (don't use speakerphone).
The Auto option for DTMFMethod negotiates RFC2833 or Inband for DTMF, and in this case, uses RFC2833 with FPL.
When Inband is used, the touch tone is sent in the audio stream as was done with traditional POTS (plain old telephone service) or as how one could imagine playing back tones from a recording.
When RFC2833 is used, an RTP event is produced that signifies the DTMF key press by the user.
Neither one of those options (or anything involving DTMFMethod) has anything to do with producing choppy audio.
Freephoneline uses RFC2833 because Inband will not work with the G.729a audio codec (due to it being a very lossy codec), and if there were an issue with RFC2833 producing choppy audio, everyone using FPL would be complaining.
https://www.obitalk.com/info/faq/Troubl ... ing-choppyYou can test the DTMF here: (804) 222-1111
That's a U.S. number.
You can test, instead, for free using these Canadian numbers:
http://thetestcall.blogspot.com/416 342 9562
250 412 5922
Inband will not work.