According to AndroidPolice, the voice calling option can be activated is by receiving a call from someone whose voice calling is already activated. Finding someone who already has it activated may be difficult though. When you receive the call, you need to close and then reopen WhatsApp. After that,you will get three tabs (Calls, Chats and Contacts). The call tab shows incoming, outgoing and missed calls,
This time WhatsApp has decided to keep the invite-window open for a much longer duration. Unlike in the past when the invite-window was only open for a couple of hours. WhatsApp has been beta-testing the voice-calling feature for quite a while now. WhatsApp have previously tested this feature, but limited the test to India, this new venture is world wide.
According to the images posted, users who receive the voice-calling feature will notice an icon for it, beside the Chats and Contacts tabs. The call interface seems to quite similar to a smartphone’s contact directory. The app keeps a record of recent WhatsApp calls as well.
(source FirstPost)