The Duo app is rather simple, with a similar interface on Android and iOS devices. You sign into the app using your phone number, and once logged in, you have a list of contacts to call pulled from the existing contacts in your phone. That's it.
When in a video call, you get many of the features you'd expect from a service like FaceTime, including mute, switching between the rear and front-facing camera, and ending the call. One new feature, however, is called Knock Knock, which is a sort of video caller-ID that lets the person receiving a call see what the caller is doing before answering. Think of it like a FaceTime video preview that's available before the FaceTime call is accepted.
Because Duo uses a phone number for authentication, the app is not available for the desktop, and you are only able to use one device associated with a certain phone number at a time. This may be a deal breaker for those who like to chat on multiple devices, but for those looking for easy video calling, it's worth a try.
We all likely know someone who doesn't use an iPhone, and Duo seems to be a good alternative for those looking for a FaceTime-like feature on Android. Duo is available from the App Store [Direct Link] and Google Play Store and can be downloaded for free.