Problems with DTMF

Troubleshoot DTMF issues when keypad selections fail in auto attendants, voicemail, conference bridges, or IVRs.

DTMF issues usually appear when menu selections fail in an auto attendant, voicemail system, conference bridge, or IVR. The most common cause is an unsupported or unreliable DTMF method.

Recommended setting

Use RFC 2833, also shown on some devices as RTP Event or telephone-event.

DTMF methods

  • RFC 2833 / RTP Event: Recommended for SIP calls. Digits are sent as RTP events instead of audio tones.
  • Inband: Sends tones as normal audio. This is less reliable on VoIP calls and can fail when codecs compress or alter the tone.
  • SIP INFO: Sends digits in SIP signaling. Use only if your PBX and destination both require it.

Troubleshooting steps

  1. Set the device or PBX DTMF method to RFC 2833.
  2. Confirm that the SDP includes telephone-event.
  3. Use a standard codec such as PCMU for testing.
  4. Disable SIP ALG if digits work inconsistently.
  5. Test again with the affected IVR or auto attendant.
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