void insertDtmf(String tones, [int duration, int interToneGap])

Source 

void insertDtmf(String tones, [int duration, int interToneGap]) {
  if (interToneGap != null) {
    _blink.BlinkRTCDTMFSender.instance.insertDTMF_Callback_3_(this, tones, duration, interToneGap);
    return;
  }
  if (duration != null) {
    _blink.BlinkRTCDTMFSender.instance.insertDTMF_Callback_2_(this, tones, duration);
    return;
  }
  _blink.BlinkRTCDTMFSender.instance.insertDTMF_Callback_1_(this, tones);
  return;
}