Consumer Reports just did a big deal on calling cards. I know in St. Thomas and St. Martin there are big signs on some payphones that say "No Calling Cards".
Consumer Reports found some phone providers that accept phone cards have funny systems for taking money from your calling card.
They pay charge in 6 minute incriments. So a 1 minute call costs you 6 minutes time, a call that is 6 minutes and 1 second costs you 12 minutes time, and some cards hit you with a 10 minute connection fee.