Voice calls on the iPhone version of WhatsApp requires that you have version 184.108.40.2064 of the messaging app, which is in beta. It can be installed only on a jailbroken iPhone. The version of WhatsApp that currently populates the App Store is 2.11.16, which of course doesn't include the calling feature.
The screenshots show small changes to the UI and a phone icon underneath a user's profile picture. Clicking on the call button brings up a dialer similar to the one on the iPhone. A "Recent" button on the bottom of the page will show a recent history of calls made using the app.
Although we have no idea when this will all be made available for iOS users, it feels like it has been a long ride since we first alerted you to the possibility that voice calling would be added to the popular messaging app. Last week, an Italian iPhone site tested out the feature and posted a video of the test. Even if you don't speak Italian, it is worth viewing just to check out the feature in action. That video is part of our slideshow below. Feel free to check it out!
source: iPhoneItalia (translated), BGR