DTMF Masking

We have a FreePBX installation that is handing off calls to several downstream switches / endpoints.

For DTMF, all of our trunk providers and endpoints are set up to use RFC2833, and ignore inband audio.

One of these downstream switches is hearing tones in the RTP stream and recognizing them as invalid button presses (a medical monitor in the background is specifically triggering a DTMF event of 9). This downstream device is also ‘set up’ to only receive RFC2833 events. I have also confirmed from captures that no out of band RTP events are being sent either via RFC2833 or SIP INFO.

The vendor is telling me that the RTP audio packets containing the tones ‘look too much like RFC2833 events’ and the device is detecting them as DTMF - even though it is not set to detect inband audio.

I am being told I should make sure the upstream device (FreePBX) is detecting any audio inband that looks like DTMF, and scrubbing it from the audio.

  1. I have looked at the RTP events versus audio that contains DTMF tones in the background, and they seem pretty different… Should the endpoint really be confusing them for RFC2833 events?

  2. Does FreePBX support masking of DTMF? I mean, it would seem like we would need to receive the call from the carrier using inband (not desirable or even possible with some of our carriers) so that DTMF in the audio channel was detected, then on trunking the call out, we would use RFC2833. But even then, it would not be reliable - which is why we aren’t using inband audio in the first place…