def reply(self, message):
        """ Sends a reply to this USSD message in the same USSD session 
        
        :raise InvalidStateException: if the USSD session is not active (i.e. it has ended)
        
        :return: The USSD response message/session (as a Ussd object)
        """
        if self.sessionActive:
            return self._gsmModem.sendUssd(message)
        else:
            raise InvalidStateException('USSD session is inactive')