I have to disagree with Steve (sort of). The cheapest way to call a European number from the U.S. may be from your home line but there is a big IF. It may be cheaper but only if you have signed up for an international calling plan.
For example, if AT&T provides your home phone service, here are the rates:
Normal rate to Germany with no special calling plan: $3.12 a minute. A calling card will easily beat this rate.
If you subscribe to the "AT&T Worldwide Occasional Calling Plan" the rate is $1.56 a minute. This costs only $1 extra a month, but you must also be subscribed to an AT&T domestic state-to-state long distance plan. A calling card will also likely beat this rate.
Sign up for the "AT&T Worldwide Value Calling Plan" and the rate does drop to $.09 a minute. That costs $5.00 a month and you must also subscribe to a domestic long distance plan. Okay, that is probably a pretty good rate, but you have to sign up for it before you go.