As reported few days back that windows phone whatsapp calling is in works today we have the 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.