Facebook probably knows a lot about you — and it probably knows a lot about the mystery people ringing your phone, too.

The company launched a new app on Wednesday intended to solve the case of the mysterious incoming call. The app, which is called Hello and is only available on Android, uses data from Facebook to tell you who’s blowing up your phone.

Of course, the feature will only work if the caller has shared his number with Facebook, and if you would normally be able to see that information. For example, if you share your number publicly, people with Hello downloaded will know it’s you calling even if they don’t have you as a contact.

Conversely, if you only share your number with Friends, those are the only people who will see that it’s you when you call. You can also block numbers easily, so if there’s a reason you’ve never shared your phone number with old Facebook Friends from high school, you can still keep them from calling.

(Side note: You can see where you stand on this spectrum by going to Settings and looking at the section titled “Who can look you up using the phone number you provided?”)

You can also use the app to make phone calls and find phone numbers for others on Facebook (including businesses). The actual phone call element is essentially the same as using your existing, built-in phone feature. Unlike WhatsApp or Messenger, which make calls over Wi-Fi or using data, making calls through Hello still requires a voice plan. The technology is simply working on top of the existing phone tool.

So why is Facebook building what amounts to a Caller ID app for Android users?

The app has a few distinct use cases beyond simple Caller ID, said product manager Andrea Vaccari, particularly for people with limited minutes or those who use disposable cellphones.

It’s easy to connect your account to any phone number through the app, so if you switch phones regularly, people will still know it’s you when you call. Also, if you know who’s calling, it can help you decide when to pick up and use your minutes (and when to send a caller straight to voicemail).

The technology might also serve as groundwork for future calling features, especially since Facebook owns two other apps that let users make voice calls. It raises the question: Why not build Hello into Messenger, which Facebook already hopes people will use to contact businesses?

“We didn’t want to force this experience on the 600 million people who are using Messenger,” said Vaccari, who also led the team that built Facebook’s Nearby Friends feature. “There is a future where we may decide to bring some or all of these features back into Messenger. That’s definitely a possibility.”

Hello is available Wednesday on Android in just three countries: The United States, Brazil and Nigeria.