Couple things try might try, as a test. Call into the IVR from s landline phone. These (older phones) will usually send continuous DTMF while a button is held down, unlike a mobile which will send a quick burst. See if that works. Try calling a DID on the PBX, then after they have answered, send some DTMF tones....can they be heard clearly? If not, check with your provider, to confirm which DTMF method they use. In the event DTMF is being sent as audio, and a low bit-rate Codec is deployed, the tone will be distorted, usually to the point that it will not be recognised.