Facebook testing an Android dialer app that gets rid of unsolicited calls
In addition, Android Police landed a screenshot of a Phone installer that mistakenly went live for a moment. It contained the app's description, which sounds something like "shows you info about who's calling and automatically blocks calls from commonly blocked numbers". Most likely, the dialer will integrate with Android to tell you which one of Facebook's 1.39 billion users with a registered phone number is calling you this very moment, even if you don't have their number in your contacts. The blocking functionality is self-explanatory.
Hopefully, Phone (if it ever materializes) will enjoy more success than the Facebook Home launcher, which flopped weeks after its launch.