Duo was introduced last year as a bare-bones competitor to more full-featured calling apps like FaceTime. It's designed to work for anyone with a smartphone across a variety of network connections. Duo, along with chat platform Allo, are part of Google's new messaging strategy, and they will be the main consumer-focused apps going forward. Duo is now the mandatory video-calling app on Android devices, pushing out Google's previous app, Hangouts. But, if you're a Hangouts fan, Google insists it's not going anywhere. As an optional download on the Play Store, it will now focus more on business and enterprise users.