It seems WhatsApp has rolled out the feature to beta testers only not to the general public and its not interested in adding any more beta users as well.
Here is the email communication happened between me and whatsapp customer care
---------- Forwarded message ----------
Date: Sun, Mar 22, 2015 at 9:55 PM
Subject: [WhatsApp Support] Your Request: WhatsApp Call
##- Please type your reply above this line -##
Thanks for your message.
WhatsApp Call is still in beta testing and is not yet widely available. As with all of our features, we are working on expanding WhatsApp Call to all of our users. We appreciate your interest but we do not plan to add any beta testers at this time.
WhatsApp Support Team
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This email is a service from WhatsApp Support.
h.duraiamuthanMar 22, 09:25I thought whatsapp supports VOIP.
But it didn't ...tapping on call leads to phone dial.I am looking forward to your reply--