I found that the difference between phone cards in Europe is pretty amazing, one card will give you a less or more than another card for the same price. If you use a payphone on the street, you loose the amount of talk time on the card, but if you call from a house or hotel/hostel phone, you get more time. When buying a pone card do not be afraid to ask questions about it, and ask to see the spec sheet on the card, the store should have it, and if the can not give you answers, go somewhere that will.
The worst one I got was in Belgium, cost 5E, and I got 14 min of talk time to Canada, what a gip, compared to the card I had before that wich gave me 40 min of talk time from a payphone for the same price!!
So make sure you shop around and look for the best deal for your needs.