Today Facebook Messenger was updated with the ability to make free voice calls over WiFi both domestically and internationally. Making calls over a cellular connection uses data. The features works similar to FaceTime Audio, which is natively baked into all iOS devices and Macs.
Facebook Messenger technically added VOIP (voice over IP) calling in January 2013, but the feature has been limited to the US, UK, and Canada until now. Facebook users with the Messenger iOS app installed can be called with a new phone icon at the top right of a conversation thread. The calling interface looks almost identical to Apple’s stock Phone app in iOS 7.1.
Messenger was recently updated with group messaging and forwarding. After WhatsApp was bought by Facebook earlier this year, it was revealed that VOIP calling is coming to that app as well in the coming months.
Source: App Store