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

def on(self, event, **options):
        """
        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
        """
        if hasattr (self, 'voice'):
          if (not 'voice' in options):
            options['voice'] = self.voice


        self._steps.append(On(event, **options).obj)