US phone cards don't work (in my experience). Go to the BTL office on front street, where you can buy phone cards that work (also available in most gift shops), and they have phones there in the office that you can use. Be advised that it is not cheap, and your minutes will run out fast. Where are you staying? They might have phones available there, but you will still have to buy the cards.
I *think* you can also rent a cell phone at BTL, but not sure the cost.
You can also have your own cell phone programmed at the BTL office to work in AC. Also not sure of the cost.
If you are just making a few calls, the cards are probably your best bet.