WhatsApp is busy testing its voice-calling feature these days. The feature is available to a limited number of people in India, and can be activated only through invite-only basis. If someone who has got this feature gives you a Whatsapp call, then you would be able to get the call feature in your Whatsapp account. However, if you want to download the APK file, you need to go to the Android section on the official website of Whatsapp.
Only Android users will be able to get this feature as of now, and they too need to have the latest version of Whatsapp, version 2.11.508, to get the feature.