Your comment makes no sense to me. If you're already on month to month, what's to stop you from signing a new contract with AT&T??? You could likely even get your # transferred to the new account. (if not, go sign up with t-mobile first, then transfer it back over and cancel the t-mobile for free).
Didn't they pass a law recently that you have to be able to transfer your number to a different carrier? I thought I heard something like that awhile ago, but maybe I'm just crazy.