After launching free calling in North America earlier this year, Facebook is expanding its VOIP service to the UK. To make or receive calls, you need to have the Facebook Messenger application for iPhone, iPad, and iPod touch installed. You can call any of your contacts by selecting the "i" button in conversation view and pressing "Free Call." Messenger supports backgrounding and push notifications, and you'll also be able to leave voice messages when your friends are offline.
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.