Try to debug it

softphone.go

@@ -94,6 +94,8 @@ func (softphone *Softphone) Answer(inviteMessage SipMessage) {
 	var re = regexp.MustCompile(`\r\na=rtpmap:111 OPUS/48000/2\r\n`)
 	// to workaround a pion/webrtc bug: https://github.com/pion/webrtc/issues/879
 	sdp := re.ReplaceAllString(inviteMessage.Body, "\r\na=rtpmap:111 OPUS/48000/2\r\na=mid:0\r\n")
+	var re2 = regexp.MustCompile("\r\nm=audio ")
+	sdp = re2.ReplaceAllString(sdp, "\r\na=ice-lite\r\nm=audio ")
 	offer := webrtc.SessionDescription{
 		Type: webrtc.SDPTypeOffer,
 		SDP:  sdp,
@@ -134,10 +136,12 @@ func (softphone *Softphone) Answer(inviteMessage SipMessage) {
 	if err != nil {
 		panic(err)
 	}
+	gatherComplete := webrtc.GatheringCompletePromise(peerConnection)
 	err = peerConnection.SetLocalDescription(answer)
 	if err != nil {
 		panic(err)
 	}
+	<-gatherComplete
 
 	peerConnection.OnTrack(func(track *webrtc.TrackRemote, receiver *webrtc.RTPReceiver) {
 		log.Println("OnTrack")