Freepbx does not hear dtmf signals from the pstn network

Freepbx does not detect dtmf sounds from the pstn network. In most cases, the problem is that the freepbx is not configured properly. In the advanced settings menu, I set the SIP DTMF Signaling to inband. Output of the dongle.conf file: [general]
interval = 10000000
jbenable = no

context = from-trunk
group = 0
rxgain = 0
txgain = 0
autodeletesms = no
resetdongle = yes
u2diag = -1
usecallingpres = yes
callingpres = allowed_passed_screen
disablesms = yes
language = en
smsaspdu = yes
mindtmfgap = 45
mindtmfduration = 80
mindtmfinterval = 200
callwaiting = auto
disable = no
initstate = start
exten = + 36209238100
dtmf = inband

audio = / dev / ttyUSB1
data = / dev / tnt1

If you are using chan_dongle, you are using a mobile network air interface. Mobile network codecs are vocoder based, so don’t encode non-harmonically related tone pairs accurately. In the uplink direction, that is handled by the air interface sending the digits out of band and the base station or mobile switching centre creating inband tones in the fixed network. I wouldn’t expect the dongle to detect upllink DTMF and send it out of band.

I checked the GSM documents recently and for a true GSM network they say handling of downlink DTMF is “for further study”. I can’t really see a good business case for other mobile phone network types to do anything different.

As such, I wouldn’t really expect downlink DTMF to work reliably, and I wouldn’t expect uplink DTMF to work well, if sent as inband.

The same issue arises if you use SIP with vocoder type codecs, like gsm or G.729.

Also note that chan_dongle is community supported and I believe there is no active maintainer. If you have to use a mobile network air interface, you would probably be better off using a SIP to mobile gateway device, but this won’t help with downlink DTMF.

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