VoIP lowers international calling rates the same way that it lowers low distance calling rates: by using the Internet to make the international VoIP call.
Basically, when you place a call from an IP phone, your call moves to the VoIP service providers network. Once the VoIP service provider finds a location close to your calling destination (like, an exit point that is geographically close), it places the call on the local telephone network.
Instead of having to pay for each network used (which is what you used to have to do), you only pay for the essential networks: the local network of the international destination you're calling and the VoIP provider's network (covered in your subscription fee). Depending on the internet service, connection quality, and VoIP network presence overseas, calls can be cheap (1¢ to 2¢ a minute for some place like France or England) or expensive (for some place like Afghanistan or rural Brazil, for example, anywhere from 20¢ a minute to much more!).
Some VoIP providers offer free in-network calling. Take advantage of this. Take your IP phone (that isn't registered to a specific location, because it otherwise you will totally screw up your E911 service) with you wherever you go and plug it into the Internet. You'll have the same phone number, no matter where you go.