Whatsapp has reportedly rolled out its calling feature that is available on running version 2.11.528 from the Play Store , version 2.11.531 from WhatsApp's website or it can be activated after users get "invited" by receiving a call.
According to Androidpolice.com, the feature gets activated after users receive a call on whatsapp from someone who already has it. After the call, Whatsapp is activated immediately after the app is closed and reopened again.
Once activated, there will be a 3-tab layout on the whatsapp homescreen showing calls, chats and contacts.
The calls tab shows incoming, outgoing and missed calls with precise timings.