def subscribe(self, number, callback):
self._subscribers[digits_only(number)] = callback<EOL>return self<EOL>
Register a virtual subscriber which receives messages to the matching number.

            :type number: str
            :param number: Subscriber phone number
            :type callback: callable
            :param callback: A callback(OutgoingMessage) which handles the messages directed to the subscriber.
                The message object is augmented with the .reply(str) method which allows to send a reply easily!
            :rtype: LoopbackProvider