Select Users Can Now Receive Google Duo Calls Without The App
Specifically, some users are now able to receive calls from others regardless of whether they have the app installed or not.
Obviously, while the person can receive a call they are unable to make a call through Duo without having the app installed.
While this previously would have meant that person had the app installed, now, it seems, it simply means they are capable of receiving Duo calls.
A point which seems to be further evident by these non-app Duo calls ending with the recipient receiving a prompt asking if they would like to download the Duo app.