First spotted by Android Police, this works only with fellow Android users; there are some other limitations that keep your entire contact list from being available to call, but it’s unclear exactly what those are just now.
That could certainly help push Duo to more people – but we’ll have to wait and see if folks care to use it. Most users of major messaging apps like WhatsApp, Telegram, and Facebook Messenger already have access to voice and video calling, so it’s hard to say whether they’ll find any reason to try another tool for the job.
Duo already comes preinstalled on certain brands of Android phones, including OnePlus. The new feature is made possible by a component in Google Play Services called App Preview Messaging that debuted in 2016, which the company previously used to allow users of its Allo messaging service to text their contacts through the app, even if the recipients didn’t have it installed.
In my brief test, I couldn’t spot any of my contacts available to call without having the app installed (I could only invite them to install Duo via a text message); let us know if you get a chance to try it out.
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Source : https://thenextweb.com/apps/2018/01/12/google-duos-voice-and-video-calls-work-even-when-your-contact-doesnt-have-the-app/363