Till today the service is available to iPhone users and restricted to the US, there no any information that when it will available on Android or Windows Mobile platform in the US. To do free voice call using Facebook Messenger user have to open conversation with friend, tap on "i" button on the top-right corner and tap the Free Call button. The recipient will get the notification for Facebook user trying to call them.
The service is very very good and addictive. The feature allows user to make a free calls without connecting cellular network. If user have good data plan, so they can make calls via internet and save the phone bills. Previously Facebook was tied up with the Skype to allow video chat on browser via Facebook. The feature makes us wondering that this feature will have limited to the messaging app or it will be also available on browser version soon?