I had unlocked my old AT&T quad band phones and I thought this would be a good way to use them as backup phones.  Come to find out, T-Mobile SIM cards work just as well in the unlocked phones and they have a much better "gold" customer deal that starts with a $100 airtime card (1000 minutes) and a $10 SIM card that also has 100 minutes.  They key is that the phone number is good for a year and is extended by adding more air time.