Actually, the best calling card on earth (an exageration?) is a Telecom Centers card for Europe and the U.S.A. I know that there is a place in Madrid on Calle Montera (right off Puerta del Sol, runs form Sol to Gran Vía). The store is called Telecom Center and the card lets you call the U.S.A or any country in Europe (just about) for 8 ptas./ minute (about 4 cents!). Even better than the Eurocity card.

Saludos,
jer...

There are many international calling cards now available in Spain. Like in the US with long distance telephone (including prepaid phone cards) rates, there is a lot of "fine print" concerning taxes, fees, connection charges, and so on. So the bold letter advertising doesn't always reflect the caller's true cost of the phone call.
I have used a card called FORTUNE which is available for as low as 1000 pesetas (2000, & 4000 pesetas cards are also available). The only limitation is that the card supposedly is only good for 60 days following its first use. When calling from a public telephone, the rate to the US from anywhere in Spain is 12 cents (USDL). From a private telephone & off-peak, the total cost of a call is around 7.5 cents a minute.
The Fortune card is available at the Telecor stand in the Corte Ingles department stores as well as in many kioskos (news stands).

The deal with the Fortune card is in order to get the best rate, you must dial a number that is not free of charge to access the prompts. Nevertheless, it's always nice to chat from your home instead of a cabina.