Missing argument 1 for _voiptwilio_call_handler()

Looking at voiptwilio_menu(), definition is:

  $items['voip/twilio/callhandler'] = array(
    'title' => 'VoIP Twilio call handler',
    'description' => 'Handles incoming calls from Twilio.',
      // Note: Since both incoming calls and Twilio responses are received
      // via the same callhandler menu, the specific call permissions are to 
      // be checked inside the call handler function itself.
    'access callback' => TRUE, 
    'page callback' => '_voiptwilio_call_handler',
    'type' => MENU_CALLBACK,
  );

But, the callback signature is

function _voiptwilio_call_handler($twilio_action, $cid=NULL) {

Arguments won't seem to be getting passed to this from incoming calls. Once setting $twilio_action to 'process_inbound_calls' manually, it takes a phone call.

Not sure, but I think something is missing.