With the addition of UK VOIP support, Facebook has essentially opened the door to free calls between its huge number of users in the UK, US, and Canada. We tested the service on its US launch and found the call quality to be "on par with competitors" such as Viber and Skype. We've reached out to Facebook to ask if it has any plans to launch the service in the rest of Europe, and will update you once we hear back.

http://www.theverge.com/2013/3/25/4144310/facebook-messenger-ios-voip-calling-uk-launch