It is more than likely a problem with your DTMF mode on the phone you are using. It could be sending the wrong tones and our system cannot recognise them. There should be a setting in the configuration to choose your DTMF mode set this to:
If it is already set to this, try setting it to:
If you have an Asterisk system with an IVR, and incoming DTMFs are not being recognised, make sure you have 'insecure=very' in your configuration settings.