code: def on(self, event, **options):
          
          if hasattr (self, 'voice'):
            if (not 'voice' in options):
              options['voice'] = self.voice
  
  
          self._steps.append(On(event, **options).obj)
docs: Adds an event callback so that your application may be notified when a particular event occurs.
          Possible events are: "continue", "error", "incomplete" and "hangup".
          Argument: event is an event
          Argument: **options is a set of optional keyword arguments.
          See https://www.tropo.com/docs/webapi/on