Facebook has grown to be more than just a place to keep up with friends, family and random strangers, but being a fully-featured social hub to communicate and share with people close to you is certainly where it plants its roots. Over the years of Facebook's growth, they have added more and more stuff to the service and eventually into the mobile applications, and even more recently they have been producing other apps that tie into Facebook in some way. The latest of these applications which looks to be going through some internal testing phases at the moment is what seems like a Caller ID app called 'Phone'.
The app was noticed by a user who more than likely received an alert accidentally to download and install Phone to their device. The message showed up in the actual Facebook application which makes sense as Facebook already promotes the existence of other apps that belong to them within the official Facebook app, like Instagram for example. The screenshot below shows options to either install or update Phone although it reportedly only directs you to an error page.
The description of the app states that Phone shows you information about who's calling, which is exactly what a Caller ID feature does for users, so perhaps Facebook is using its massive library of user data to provide the phone number information to anyone with this app installed when they receive a call from someone whose number is listed on their profile. It also states that it has the ability to block calls from commonly blocked numbers. This isn't terribly descriptive, and leaves no detail on how the function works, so we could only assume that Facebook has a database of numbers that are known to be blocked on a more consistent basis and uses that data to automatically block those calls when they're incoming for Phone users. However it works, Facebook has made no mention of the app officially so it may never end up launching at all, and it wouldn't be the first time we saw some testing for applications that have yet to see the light of day from the social media behemoth.