The easiest way is to go into one of several internet cafes in SP. Most, if not all, by now have the protocol that allows clear long distance calls to the States. You give them the number, they dial it, and hand you a cordless phone and you are good to go. The price has been dropping in the last year and it seems that it is somewhere between .60 and .75 US per minute. This works, therefore a far better option than a calling card that may or may not.