Pedro is correct. BTW Interesting fact about ATT iPhones
Interesting to know (and ATT does not advertise this) but any iPhone that has had it's 2 year contract fulfilled can be taken to ATT where they will happily unlock it for you. I don't know if you can buy an iPhone 5 full price and have them do it, but it would be interesting to find out. I don't know but I will ask my ATT rep about this next time I see them.
Anyway, then you can use it anywhere you go just as long as the cell network uses the frequencies the antenna in the iPhone model you have uses.