onTransferred method
Handle transfer response event raised by library and route it to matched call instance
Implementation
void onTransferred(int callId, int statusCode) {
  _logs?.print('onTransferred callId:$callId statusCode:$statusCode');
  int index = _callItems.indexWhere((c) => c.myCallId==callId);
  if(index != -1) _callItems[index].onTransferred(statusCode);
}