On Android devices, an incoming call shows up like a normal phone call with the calling party’s full screen picture. On an iPhone, the call is denoted by a push notification. Calls can only be made between users who have the Facebook or Facebook Messenger app installed on their phone. It is not available on the web version of Facebook Messenger.
Facebook first introduced the feature for iOS users in US and Canada in January 2013. It was extended to Android users in April 2013.
Interestingly, the introduction of the feature comes shortly after mobile messaging app WhatsApp, which was recently acquired by Facebook, announced plans to introduce free voice calling.