South American cell phone use

[silvarios]

If you want access to 3G, unlocked AT&T phones will probably function (assuming coverage), but not T-Mobile. T-Mobile in the USA uses AWS, not many carriers use this frequency (all in North America I believe). Either way, pick up a local SIM card so you don't have to pay extraordinary amounts of money to use your phone in another country.

Or do as Ombligo suggested and get a local phone for next to nothing.