I would be very careful using phone cards purchased in the U.S. and used in other countries. Most distributors will not allow callbacks on their cards, due to the fact that the call is originated through an international switch. Making it difficult to guarantee rates, or make calls at all.
I owned a phone card vending business here in the midwest for 10 years. When people ask me about calling back from other countries, I tell them to purchase a card when they get to the airport, or hotel. That way you know it works from that country.