Google Duo features:-
Straightforward interface without a long process. Pick up your any contact, and get in connect.
Knock Knock: -
If anyone calls you, you can see her/his name before pickup the call.
High-quality video: -
Google tech team especially work on to give excellent video quality even in the low internet.
You can call to any of your friend who is using Google Duo on Android or iOS platform. According to some news, only audio calling feature is coming so soon.