Vonage is a VoIP service provider which lets you make over an internet connection. Unlike Skype and many other VoIP services, Vonage typically works using a real phone which you connect to your home internet connection using an adapter. But that service will cost you, with monthly plans ranging from $10 to $35 per month.
This week Vonage launched a new free service for mobile devices that lets you make free calls to your Facebook contacts.

Here’s how it works. You install the Vonage Mobile for Facebook app on an iPhone, iPod touch, or Android device. Then you login to your Facebook account. Vonage will scan your contact list to find anyone else who has the Vonage app installed. When they’re online, you can make free voice calls to those contacts over an internet connection — without using up any of your mobile minutes (although data use charges may apply).
You can also use the Vonage app to send instant messages to your Facebook contacts — even if they don’t have Vonage installed. The app has a call log feature, and a messages tab.
Vonage Mobile for Facebook is available as a free download from the iTunes App Store and the Android Market.