The read-only property RTCDTMFToneChangeEvent.tone returns the DTMF character which has just begun to play, or an empty string (""). if all queued tones have finished playing (that is, RTCDTMFSender.toneBuffer is empty).

A string with the DTML character playing, or the empty string.

This example establishes a handler for the tonechange event which updates an element to display the currently playing tone in its content, or, if all tones have played, the string "<none>".

dtmfSender.ontonechange = (ev) => {
  let tone = ev.tone;
  if (tone === "") {
    tone = "<none>";
  }

  document.getElementById("playingTone").innerText = tone;
};

• WebRTC
• Using DTMF with WebRTC
• tonechange
• RTCDTMFToneChangeEvent