I have never used the phone cards, but did notice a lot of payphones with signs that said "NO PHONE CARDS" in St. Thomas and St. Martin.
Also, Consumer Reports in October did a sidebar story on phone cards, and noted that some phone carriers use odd ways of calculating how long you were on the phone. Some bill in 8 minute blocks and round up. So any call less than 8 minutes costs you 8 minutes off your card, a call one second over 8 minutes would cost you 16 minutes off your card.
And now there is a big legal battle, the FCC wants to force long distance companies to collect taxes on calling cards........you know, the 911, and Lifeline taxes you pay at home. ATT says that could boost the cost of a call made with a pre-paid phone card by up to 15 cents a minute. So, buyer beware.