It's been a while that people are waiting for the Whatsapp calling feature. There were rumors and so many tricks going on internet. After Facebook took over whatsapp this a major update from them. The new calling feature are rolling out officially.
Now you dont have to wait for the invitation from others to enable the calling feature in whtasapp. The update is rolling out check for it. You should be using the new Whatsapp version 2.12.5 as mentioned in Google play about the version. So update your whatsapp and enjoy your calling with Whatsapp.
If you have any problem regarding this feel free to comment.