How do I use WhatsApp Calling?

WhatsApp Calling lets you call your friends and family using WhatsApp for free, even if they’re in another country. Currently, WhatsApp Calling is available on Android, iPhone and BlackBerry 10.
WhatsApp Calling uses your phone’s Internet connection rather than your cellular plan’s voice minutes. Data charges may apply.
Please note: you can’t access 911 and other emergency service numbers through WhatsApp. You must make alternative communication arrangements to make emergency calls.

Placing a WhatsApp call

To make a WhatsApp call, simply open the chat with the person you want to call and tap on the phone button on top.

Receiving a WhatsApp call

When someone calls you, you will see an incoming WhatsApp call screen. You can tap and slide the green button to answer the call, or slide the red button to decline the call. Alternatively, you can tap on the message icon on the call screen to decline the call with a quick message.
Learn how to use WhatsApp Calling on: iPhone | BlackBerry 10.
Cheers,
WhatsApp Support Team