The Dual Tone Multifrequency (DTMF) tones are either not received or not understood. The devices must be able to receive or respond to DTMF to process calls or to navigate menu systems. One example is voice mail systems. The issue may be due to a configuration on a gateway, signal strength, or connectivity issues.
To resolve the problem, refer to these steps:
Make sure the dial-type is set as DTMF on the router under the voice port, as shown in the following example:
The distortion of DTMF tones is more likely when compressed codecs are used. From Cisco IOS Software Release 12.0(5)T and later, the DTMF tones can be sent out-of-band between Voice over IP (VoIP) gateways. For more information, refer to the Solutions section of Troubleshooting Inability To Break Dialtone, and scroll down to Solution 4.
The signal strength may be too high or too low on the voice port, causing distortion on the in-band transmission of tones. The signal strength should be changed depending on the current configuration. For both parameters below, the default is 0.
Router(config-voiceport)#input gain ?
<_-6 _-="_-" _14="_14"> gain in db
Router(config-voiceport)#output attenuation ?
<_-6 _-="_-" _14="_14"> attenuation in db