I think T-Mobile do a service called 'paired Digits' or something like that. You can have a main phone and a second phone that shares the number and the data service. But only the main phone is able to receive security codes to get you back into your account's etc if you get locked out of a service/app etc.
It would be uber cool if Virgin offered this service to its unlimited customers 😉
That way I could be reached no matter which device I take out with me 🙂