What needs to be activate whatsapp calling features on mobile

You need to first get latest version of whatsapp that is 2.11.561. After you’ll have to ask a user who has the feature to call you. Once you’ve received the call, close the app and reopen it. You should now see a new screen with three columns, including one for calls. 

Make sure.. you have latest version of Whatsapp. 2.11.561 
Version may differ according to device brand/model/make..

Edit 2:
This feature will only work on Android devices.

Edit 3:
iOS and Windows users can only receive calls made from Android device via Whatsapp.. not vice-versa.

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