Free internet calling has become a reality with the new (April 02 2015) WhatsApp updates. If you have the Call feature, then you'll see the three columns as above here... and you can start calling any of your friends who are also enabled...
but that's not all! You'll also find a WhatsAppWeb link that you can scan with your app to link your phone to your computer... and then send whatsapp messages (including photos) directly from your computer to your WhatsApp friends.
In short, WhatsApp has suddenly become essential.
WhatsApp has already revolutionised the chat/messaging world and now it's directly challenging both cellular and VOIP.