Google Duo – This application has already become a hit, although it happened only recently, it not only surpassed its counterparts, but literally left them far behind. All this is due to its non-standard functionality. While other applications for video calls show two people at once, this application has the Knock-Knock function. It’s such a knock on the door, namely at the time of the call, the person who takes the call can see what the caller is doing, until the caller sees anything. And so you can decide to accept the call or not.