I have a Samsung Captivate, noticed Christmas night while in my basement (where I do not get service) the lock screen had T-Mobile in the top right corner. This is a stock phone never rooted or anything like that. Should an AT&T phone display T-Mobile? LOL
AT&T and T-Mobile have mutual roaming aggreement that phones from one carrier can roam to the other when the phone can't connect to its home carrier. So, it sounds like you have no AT&T signal in your basement but T-mobile signal is available.
So would you rather have no signal at all or let it roam to T-mobile?
I guess that works for me, but regardless of the carrier you can see from the signal indicator that I didn't have service at all. I found it interesting that my AT&T phone was displaying T-Mobile, I have never noticed that before.