That is the easiest way. Codetel sells cards, as do others. they are easily obtained. Another good possibility is to call from a Codetel office. At least one in every town and city of any size. When finished, pay the person at the desk.
Harpua, I bought a prepaid calling card and never used it. It was easier to call from the various "call centers". It was easy and cheap calling to the US and all over. Just follow the instructions at the center and pay when done.