To get this feature, Android users need to update to the latest version and then find out a friend who already has the feature. After this, your friend, who has who has that facility should call you up to activate the facility.
In this the app’s bar at the top features three primary tabs, one for text-based chats, one for contacts, and a new one for calls. Moreover, The WhatsApp calling feature doesn’t require Android Lollipop. It also works on Android KitKat. For the better experience, use the Version 2.11.531, which is available directly from WhatsApp website
If some one calls a user who has the older version of the app, the message tells the user that the app needs to be updated. The version of WhatsApp that supports calling is 2.11.528. Initially, when WhatsApp was testing this feature in a limited way, it worked only on Android phones running the Lollipop.
Source : Gizbot