WhatsApp seems to be beta testing a voice calling feature in it's software, with images from the interface appearing on Reddit. While the post has since been taken down, and the update doesn't seem to be reaching all users either.
The feature appeared as part of the 2.11.528 update, that the Reddit poster said he downloaded off WhatsApp's official website, but the the same is currently unavailable. He also posted the video below to YouTube, detailing how the system works. WhatsApp have not made any official announcements for the update, nor have they detailed the changes in their version release notes, which still refer to 2.11.527 version.