WhatsApp has included the voice calling update in the latest version of the Android app. The feature is available through the 2.12.19 update and for now has to be downloaded through the company’s website, although should soon be available through the Play Store as well.
WhatsApp started rolling out the voice calling feature a while back but instead of making it available to everyone at once, they used sort of an invite system, where a user who already has the feature has to call you for you to get the calling feature. And this worked within a time window, so if you didn’t manage to get it activated then you’d have to wait for the next time window, whenever it arrived.