Wow I just got back from Colombia. Get the phone unlocked by ATT a phone that old should be OK. When you arrive in Colombia any employee at Tigo will set your SIM in less than 5 minutes and get you going.

They do have iphones 5 and 4s so the 3gs is not the rave at all but you can get @ 150 usd more or less. I kept mine for intl travel But it is so slow in comparison to the 5. Like driving a Smart car after being used to a BMW