Call can be received on the phone without Duo app. Incoming call will show on the screen instantly just like for the Duo users. All other features of Duo call also work similarly. You can receive, reject, mute the mic or change the camera.
At the end of the call, users are prompted to install Duo if they’re interested in starting video calls of their own. Message at the bottom of the screen allows you to block a caller from contacting you again.
After reading this you might be thinking that how it is possible to get Duo call without installing Duo app on phone. Google made it possible through a feature of Google Play services called App Preview Messaging, which was launched alongside Allo back in 2016. App Preview Messaging was made available to some developers through an early-access preview program with the statement that it would open for all developers of messaging apps, but no other developer has come up with such feature yet.
Through this new feature, Google has made Duo calls more accessible to the people. It also solved the disappointment of the message showing “none of my friends use it.” However this feature is not available to all users yet. iPhone users and all other non-Android phone users can’t avail this facility of Duo calls. Google has wisely used this feature to promote Duo app.
We can expect some more developers come up with this feature in near future. People are only reachable through App Preview Messaging if their phone number is registered with their Google account. You might suggest this to your family and friends. Also make sure that “App preview messages” is enabled in the Google service settings, which should be enabled by default.