WhatsApp has matured of late and has seen wide adoption across various mobile platforms. They even have apps for Windows PC and Mac along with mobile apps for major platforms and a Web app.
WhatsApp has also added Audio and Video call features (which are quite similar to FaceTime Audio and Video), and the recent version are integrated deeply in iOS and Android. Audio and Video calls can be received just like regular calls, even when the app is not running (but an active Internet connection is required).
It is mandatory to run WhatsApp using a mobile app, as the new account registration is done using an active mobile phone number. The WhatsApp mobile apps needs to be running for Desktop and Web app to work.