Hopefully this feature will be offered outside the US in the near future!
Originally posted on Gigaom:
Starting Wednesday users can now use Facebook(s FB) to make a voice call to another Facebook user. Facebook is soon rolling out an update to its Messenger iOS app with the service, which is free and will work over Wi-Fi or a data connection when it becomes available in the U.S.
The Verge noticed the new button first, and Facebook confirmed the service would roll out starting Wednesday. The Messenger app will get a new button that will make calls. It’s voice-only, however; this is not (yet) a service in direct competition with Apple’s own FaceTime app for iPhones and iPads.