So if you're looking for an alternative to the phone number, start with something more easily replaceable. Hardjono suggests, for example, that smartphones could generate unique identifiers by combing a user's phone number and the IMEI device ID number assigned to every smartphone. That number would be valid for the life of the device, and would naturally change whenever you got a new phone. If you needed to change it for whatever reason, you could do so with relative ease. Under that system, you could continue to give out their phone number without worrying about what else it might affect.