The news had first posted by the Android Police, when they saw a notification of the Phone app on Facebook itself which may be testing internally as it says [FB-ONLY] on it. Till yet there is no news about this app or the feature, so company is silently testing for a new feature on Facebook.
Generally, Facebook does a major interface updates to its application server-side, so users occasionally get seemingly random glimpses of new features or UI designs before the official launch. This glimpse suggests that someone committed an ‘innocent’ mistake and hence the notification goes on.
Many of the facebook users have saved our phone numbers on our facebook accounts, so facebook Phone app will provide a detailed information about the caller even the number is not on your list. Phone app will block calls from commonly blocked numbers and show you detailed information about incoming calls.
It yet there is no update from the facebook side, but it does seem like Facebook is at least testing it.