You know the WhatsApp calling feature that has been rumored for almost a year and supposed to roll out by the end of 2014? Well, it seems to be here, a few months late, and with a major caveat for now.
Reddit user pradnesh07 has come across the feature by receiving a WhatsApp call from a friend. That's how the option is presumably activated for each user, in an invite-like system where you have to get called by someone who already has it enabled. Here is the app's updated UI and how the calling interface is supposed to look.
Pradnesh07 went on to call a few users too, trying to trigger a cascade of invites from his device. For now, he (and others) is reporting a hit-and-miss situation when trying to enable calling for his friends. You might have to grab the latest WhatsApp apk from the service's website, you seem to be better off on Lollipop than previous Android versions, you could try this terminal trick if you're rooted, and you may have to force close or reinstall the app if you're lucky enough to receive a call, but none of it guarantees that the feature will get activated for you. I still haven't managed to do it on my phone.
So for now, all we can say is that WhatsApp isn't content with rolling out its Web interface at the start of 2015. Calls are coming, but they don't seem to be ready for primetime just yet. I guess we can always go to Skype, Hangouts, or Viber. Bummer.
Source: Reddit. Thanks, Redditguy!