Facebook has been slowly rolling out VoIP calling to its Messenger app in an effort to make it the single, unified source for all your communication needs (before Google can). Today, it apparently took another baby step forward by granting UK users of the mobile app the ability to connect for free (minus data charges where applicable) to anyone they're friends with.
At the moment the service doesn't seem to have rolled out to all users yet, and it may even still be a bit buggy (the slow expansion is partially to conduct tests, after all). However, the important thing is it's coming. You won't need to update the Messenger app to get the new functionality. It should just appear when you have access to it. If you don't have it, you can grab it via the widget below.