We have heard about this a couple of times about WhatsApp testing and planning to test the Call Feature in Windows Phone, but today we have a screenshot as a confirmation.
Similar to Android, you get a new Call Tab which will keep track of the recent call history indulging incoming, outgoing and miss calls. Those who are wondering if this features gets enabled by a miss call or a call, then the answer is no. You need to get the new build which is right now is 2.12.38. As of now the calling feature is pretty stable as either its not connecting or takes time to connect.
Thanks Its_VipiN & Abhi