In an attempt to push Duo's voice and video calling, Google is allowing users to ring contacts who don't have the app installed yet. Initially, this only works with Android users, also with some limitations.
However, major competitors like WhatsApp, Facebook Messenger and even Telegram have already promoted their own voice and video calling, and all have succeeded to their own extents. It's difficult to say that Google will be able to reach those already-pleased users to ditch their favorite app in favor of Google's.
But still, Google has an advantage.
Duo comes preinstalled on some brands of Android phones, including OnePlus.
Those who tested the feature noticed that it works just like when the user has Duo installed. Recipients can do things like answer/hangup the call, mute it, and switch cameras, just like usual.
What's more, they can also block a caller from contacting them again. However, if the someone without Duo installed wants to make a call, they have to install the app first.
Google in making Duo capable in calling those that haven't got the app installed, is made possible by a component in Google Play Services called App Preview Messaging that debuted in 2016. Previously, the company used to allow Allo users to contact those that haven't got the app installed on certain Android phones.
This could certainly help push Duo (and previously Allo) to more people.
Google Allo supports App Preview Messaging since launch, albeit with a different UI. And its compatibility is also part of the stock Android Dialer.