Facebook is now rolling out a new Facebook Messenger for iOS feature that lets US users call other US-based Messenger for iOS users.  So, if you’re on a low-minute phone plan or suffer from bad cellular reception, you can call your Facebook Messenger friends using your phone’s data plan or using a Wi-Fi network.  The new feature will show up automatically as it’s being rolled out, as long as you are currently using the latest version of Messenger.

In order for Facebook Messenger calling to work, the person you’re calling must also have the feature added to their Facebook Messenger.  Although the calling feature will be rolled out to all US users, you and the other user must have the latest version of Messenger installed for the feature to be added.

Making a call is easy: just open a conversation with that person, tap the “i” button in the top-right corner, and tap ‘Free Call.’  After you initiate a call, the person who you’re calling will get a soundless notification on their screen that tells them you’re calling, and they can accept or decline.

For now, Facebook Messenger’s calling service is voice-only, but if/when the app adds video calling, it could pose a serious threat to Apple’s FaceTime, Skype, Viber, and many other competitors.  Although Facebook and Skype have partnered since 2011 to offer Skype calling to all Facebook.com users, Skype apparently did not help Facebook with the calling feature in Messenger, and according to Wired, Skype also denies any involvement in possibly adding video calling to Messenger.

For now, Messenger’s calling feature is limited to US iOS users, and Facebook has not provided a timeline for a rollout to Android users and those in other countries.  We’ll let you know as soon as they do.