The tonechange event is sent to an RTCDTMFSender by the WebRTC API to indicate when DTMF tones previously queued for sending (by calling RTCDTMFSender.insertDTMF()) begin and end.

To determine what tone started playing, or if a tone stopped playing, check the value of the event's tone property.

This event is not cancelable and does not bubble.

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("tonechange", (event) => {});

ontonechange = (event) => {};

An RTCDTMFToneChangeEvent. Inherits from Event.

In addition to the properties of Event, this interface offers the following:

RTCDTMFToneChangeEvent.tone Read only

A string specifying the tone which has begun playing, or an empty string ("") if the previous tone has finished playing.

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>".

This can be done using addEventListener():

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

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

You can also just set the ontonechange event handler property directly:

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

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