In early January, Facebook began testing free calling over Wi-Fi and cellular data for all Messenger for iPhone users in Canada, and said that a US launch could be coming soon. Apparently, the test went well — a new free calling button has appeared in the app. Facebook has confirmed to The Verge that the feature began rolling out to US users today, and requires no update through the App Store. To make a call to another Messenger for iPhone user, all you need to do is open a conversation with that person, tap the "i" button in the top-right corner, and tap Free Call.

If you live in the US, you can now call other Facebook users for free over Wi-Fi or using your phone's data connection

What this means is that if you live in the US, you can now call other Facebook users for free over Wi-Fi or using your phone's data connection while you're on the go. When you call someone, a push notification appears on their screen that says "Ellis Hamburger is calling," for example. The feature is especially critical for people with bad cell service at work or at home, and for those who want to conserve cell phone minutes. It's also a huge step for Facebook — which with a single feature emerges as one of the largest communities of VoIP users in the world. After a few tests, the call quality sounds very good, and is certainly on par with competitors Viber, Vonage, and Skype — which have had the feature for some time, but all have much smaller user bases.

Facebook has had a partnership with Skype for video calling inside Facebook's website since summer 2011, but Messenger still notably lacks video calling. Facebook offered no more details or information about an international rollout, VoIP calling through its Messenger for Android app, or VoIP calling through its website, but we'd bet these things are coming soon.