Google Duo

Google Duo - This app has already become a hit, although it came out recently, it's not just surpassed their counterparts, and just left them far behind. All this thanks to its non-standard functionality. While other apps for video calls show two people, this app has the function of Knock-Knock. It's sort of a knock on the door, namely in the time of the call, the person who takes the call can see that who is calling while the caller saw nothing. And so you can decide to accept the challenge or not.