Your mate also has to have the Vonage app installed, but unlike Skype, you don't have to sign up for an account outside Facebook. You just sign into your Facebook account in the app to see who's online and give them a ring.
We got our hands on Vonage last week and we think it's the killer phone app Facebook has been looking for, because you don't have to faff around with creating a new account, dialling an access code or making VoIP calls from your computer. And since it rings when the app is closed, you're always available for your Facebook friends to call, even if they don't have your number.
The app supports calling over Wi-Fi and 3G, although you can turn off the 3G option if you can't spare the data.
Vonage is a big name in VoIP from the US, so we think call quality should be decent too. The app comes out in the iTunes app store and the Android Market today, and we'll update this story with links when we have them.
In the meantime, take a walk though the photo gallery above to check out some of its features.
Update: Download Vonage for iPhone and Android from Vonage's Facebook app page.