Android WebRTC cant send DTMF

Question

In my Android WebRTC client to send DTFM tones I use code like this

val audioSource = peerConnectionFactory.createAudioSource(MediaConstraints())
val localAudioTrack = peerConnectionFactory.createAudioTrack("audio", audioSource)
peerConnection.addTrack(localAudioTrack)
peerConnection?.senders?.firstOrNull()?.let {
    it.dtmf()?.insertDtmf(code, 400, 50)
}

But it seems tone does not reach a peer, and there is erro message in logcat
dtmf_sender.cc E (line 126): InsertDtmf is called on DtmfSender that can't send DTMF.

No matter what device I use.
Why could it happen?

Answer 1

There are multiple reasons why this could happen, one of them being that the other party in the WebRTC connection does not support the RTP telephone-event

Also, check this example: https://developer.mozilla.org/en-US/docs/Web/API/WebRTC_API/Using_DTMF (haven't tried it myself though)

Pay attention to this:

"Note, however, that although it's possible to send DTMF using WebRTC, there is currently no way to detect or receive incoming DTMF. WebRTC currently ignores these payloads; this is because WebRTC's DTMF support is primarily intended for use with legacy telephone services"

Answer 2

I have found, problem is that peers are not connected via RTP. I mean WebRTC did not found yet suitable route, basing on ICE candidates gathered, to pass audio traffic.
As only route is constructed, traffic goes on, and sender comes ready to send DTMF tones.
To be ensured that peers are ready to trancieve media, you may look on connection state in PeerConnection.Observer.onIceConnectionChange(), and get sender when state comes to "CONNECTED".