Probably t-mobile at the moment, but you would still need to buy an initial fill of $100 to get the best rate on your minutes and to make them last a year, though you can buy the fills/refills slightly discounted at places like callingmart.com. (they offer 5% off on the web site, but they often have additional discounts that they announce on their facebook wall and via their twitter account. You can usually get the $100 refill for $92 or so if you wait for the sale price. Otherwise it's $95.)
If you top it up before the year is up, they extend all your time for another year. They send you a message on your phone with that info when your minutes are close to expiring.
Unfortunately, AT&T is trying to buy t-mobile, so there's no telling how long the t-mobile prepaid plan will last, since it's much better than the AT&T 2 GO prepaid plan.
If you let the SIM expire, you do lose the phone number.
-David
Last edited by LIH Prem; Aug 3, 2011 at 3:59 am