When you use your cellphone to call abroad use this service:
http://www.phonetonetele.com/
They offer cellphone calls to the USA for just €0.04 to €0.07 compared to the normal price of around €0.55. All you need to do is call their access number, and then dial the international number.
BEWARE: MCI, AT&T cards etc.. will NOT work on european cellphones as these companies would need to pay the extra tarrif for mobile originating calls. I've been using phonetonetele for some time now and am extremely happy with it. Call quality is great.