Few hours ago , Whatsapp rolled out an update which now offers the voice calling feature to everyone! .Before now you needed to have an invite – a call from a contact who had the feature enabled – to be enabled to start using WhatsApp voice calling , but now ,This feature is available to everyone with the latest version of whatsapp.
To access this feature , all you need to do is download the latest version Of whatsapp. Download Here.
NOTE : It may take a while before everyone gets the update. so if you dont see it now check back later.
i saw in a post where someone is saying that it is for just some selected people. thanks for throwing more light.