According to ABC News, in the app, select a friend, then the "i" icon in the upper right hand corner, and you will then see a "Free Call" option.
Facebook confirmed to ABC News that users will not have to update the app for this feature, as it will automatically appear for those who already have the app.
As with Skype or other mobile VoIP (Voice over Internet Protocol) services, you can make the calls over Wi-Fi or a 3G, 4G or LTE connection.
This is just one of many mobile announcements Facebook has made in the past few months.
According to the report, it upgraded both its iPhone and Android apps to make them faster and it released a new Poke app, which destroys messages 10 seconds or less after you look at them.
Facebook didn't make a big announcement about the voice calling service, but it's a very big move for the social networking company, the report added.