This picture describing Rebtel's recent offer picture pretty much says it all here, but as usual, the fine print is what you need to read.

Rebtel assigns your international contacts a "local" number. This local number essentially "forwards" the call-using VoIP-to your international contact. The cost savings in some cases is substantial. A number of other companies now employ a similar technique, but Rebtel was one of the first.

Even so, $30 in credit for the price of $10 is worth checking out-with the restrictions in mind, of course.