Duo

Usually, apps that give users the option of making video calls are also providing a host of numerous other functions as well, but Google Duo is different in this regard. The app uses only the frontal camera for calling, and you can only use the contacts in your address book.
 
Google Duo can be used both on mobile data connections and Wi-fi, but the latter is recommended since it involves quite a bit of traffic. Also, the app can only be used from a phone that has a sim card or a phone number attached to it.
 
The application has a very handy feature called “Knock, knock” that lets that user that’s being contacted to see who’s calling before picking up. The ability to block numbers is also provided, along with a list that makes it easy to manage them.