Facebook has just updated is Messenger app on iOS and Android to version 4.0 in which it (among bug fixes and UI changes) has added the option to make VOIP calls. You can now make calls over Wi-Fi and Cellular with your friends for free (even if they are in another country), in order to do so, your friends will need to have the Facebook Messenger app installed.
“Talk as long as you want and stay in touch wherever you are”
Facebook Messenger isn’t the only app that has VOIP calls, last months images of how WhatsApp will also integrate VOIP calls were leaked. The interfaced that will be displayed when you are making a call using the Facebook Messenger app looks similar to the native iOS 7.1 Calling UI. The voice calling feature wasn’t the only change made in this new update, on Android you can now also create group chats, forward messages (these features was already available on iOS) and create shortcuts on your homescreen to your favorite conversations.
See Also: Top 3 secure VOIP apps for Android