It works like this: you install the Vonage Mobile application on your phone and log in using your Facebook account. Once you do that, it will show all your friends that also happen to have this application installed. You can then call them and talk to them, over both 3G and WiFi. And it’s free – even if they are in a different country – because the call goes over the internet.
The app is far from perfect. For instance, it does not yet support multitasking on iOS 4. But it works, and it’s free, and it’s convenient. Hopefully they work the kinks out soon. Try it out, and if I am your Facebook friend, then I will automatically show up on your contacts list. However, be careful if you are using this on 3G and have a limited data plan.