Image credit – Android Police
If you wanted to call someone using WhatsApp’s voice/video calling feature, they would also need to have WhatsApp installed. This is same for other VoIP/messenger apps like Skype, Viber, and so on. This was also true for Google’s Duo app, at least until recently where it seems that Duo is now allowing users to make calls to non-Duo users.
This was posted on Reddit (via Android Police) in which the poster claims that they are able to make Duo calls to other user who don’t have the app installed on their phone. The poster claims that Duo’s list of contacts showed his friend’s name as having Duo installed, despite the fact that his friend did not.
The user was then able to make the call to his friend and have access to the same Duo call features as you would during a normal Duo-to-Duo call. It has been suggested that this was made possible due to the App Preview Messaging that was introduced to Google Play Services in 2016.
At the moment it seems that this feature isn’t available to all users and only limited to those on Android (since iOS doesn’t have App Preview Messaging). No doubt that being able to call a non-Duo user through the app will be pretty convenient, but no word on when it is expected to roll out in full to all users.