Google Duo is a very simple video calling app for Android and iOS.
Duo is all about ease of use. There's no separate account to create, no need to build a new contact list: the app can use your phone's contact list already, and start a video call with a click.
Duo's "Knock Knock" feature means users will see live video of any caller before they pick up, giving a quick idea of where they are and what they're doing.
The app does its best to keep you connected, seamlessly switching between mobile data and wifi as necessary, and automatically adjusting resolution to match available bandwidth.
Google hasn't forgotten about security, either-- end-to-end encryption keeps snoopers out, even on insecure wifi hotspots.
A likeable video calling app, easy to use but with some smart engineering under the hood.