When one dials an international extension, Truphone detects the number and offers the option of connecting via its program. Once the user chooses to do so, Truphone dials into a local server, and from there, it calls the international number and merges it with the local call, thus using your local minutes. Of course, you do have to pay Truphone per minute as well, but it is significantly cheaper than a carrier's international calling prices.
The software is available for download at Truphone's website, or at Handango's online store.
source: Truphone via SlashPhone