On Wednesday Facebook added the ability to make voice calls to its stand-alone Messenger app. The feature is only available for iPhone users, and only works if both the caller and the recipient have Facebook Messenger app installed on their phone. While the feature is definitely good news for Facebook users who use the app, Facebook isn't the first company to offer free voice calls over data networks using your iPhone. Check out the video above for a closer look at how to make calls using Facebook Messenger, as well as a look at some of the competition.