Discussion going on on Reddit suggests that some users have got the call functionality activated on their WhatsApp Android app, although there is no clear cut method using which you can get it on your own. Not by grabbing the latest APK from WhatsApp’s own website, neither by any other trick.
The only feasible system might be to get a call from someone who’s got it activated, whether that’s under slow gradual rollout or by an error on WhatsApp’s part. What we’re seeing above is WhatsApp new Interface when calls have been activated. There is nothing confirmed, when it’s region locked, or by Android OS.
Seems the voice calling isn’t quite ready for the prime-time yet, even though the rumored 2014-end release is long gone.
We’ll keep an eye on further developments and will be sure to get you the news. Keep watching this space. Meanwhile, grab Viber, or Skype, they too do the job well.