onDtmfReceived method
Handle received DTMF tone
Implementation
void onDtmfReceived(int tone) {
if (tone == 10) {
_receivedDtmf += '*';
} else if (tone == 11) {
_receivedDtmf += '#';
} else {
_receivedDtmf += tone.toString();
}
notifyListeners();
}