Facebook Messenger for iPhone got free voice calling feature in the US and Canada back in February 2013. It rolled out for Android users in Canada in March, it rolled out in the US and in 24 countries in April last year. You can make calls only to Facebook Messenger users on iPhone and Android over 3G or WiFi. It is not available for Facebook Messenger for Windows Phone that was launch recently. The Free call option is grayed out if you can’t call the person. This was first spotted by AndroidOS.
You can even call from Chatheads and also use speaker phone. It even asks you to rate the call quality after you finish the call. The call quality is good if you have good network or WiFi connectivity. Conference call is not available yet. BBM 2.0 for Android and iOS got voice support recently. WhatsApp is expected to bring voice support in the second quarter of this year.
Looks like Facebook is gradually rolling out the calling feature globally. Did you try out the new calling feature on Facebook Messenger?