Facebook has quietly rolled out a new feature on its ‘Messenger for iPhone’ app that will allow users to make free VoIP-based calls to friends on the social network.

The service – currently available only in North America – is seen as another possible threat to operator voice income, as well as a potential new rival to established VoIP services such as Skype and Vonage. It works across Wi-Fi and mobile data networks.

Facebook is said to have been testing the service in Canada since the beginning of the year. According to The Verge, the initial trials have gone well and the feature began rolling out to US users yesterday.

Enabling the feature requires no update via the App Store. To make a call, users are required to tap the ‘i’ button in the top-right corner of the app, and then tap ‘Free Call.’ Recipients need to have the standalone Messenger app installed on their iPhone.

It is not yet known when the Android version of the Messenger app will get the new calling feature or when the service will be rolled-out to international markets.