For calling back to the USA, I think the cheapest and most convenient way is to use the toll-free number in the country you are calling from to access your long distance carrier at home (ATT,MCI,Sprint,etc.) and then dial direct with credit card or with the help of a US operator, very convenient and no language dificulties,etc. also a way to get mileage for the calls if you are signed up for that program. The toll free access numbers from each country are listed almost daily in the Int.Herald Tribune in ads for AT&T, MCI, etc.

You, however, cannot use this for calling within Europe so you may need to get a calling card anyway, just not in such large such a large denomination.