Best of Breed

The TruPhone app, which is available for multiple platforms and mobile OSes is by far the most polished, stable and best performing I have seen. This company also offers a SIM product that offers unique features well suited to people who travel extensively or are away from home for travel, school or business.
Support is excellent and the company is growing in size and reputation. Give it a try.

DavGreg about Truphone – Free calls & messages, local or international, v5.4.5