WhatsApp Calling lets you call your contacts using WhatsApp for free, even if they’re in another country. Currently, WhatsApp Calling is available on iPhone (on iOS 7 and later), Windows Phone, and Android.
WhatsApp Calling uses your phone’s Internet connection rather than your cellular plan’s voice minutes. Data charges may apply.
Note: You can’t access 911 and other emergency service numbers through WhatsApp. You must make alternative communication arrangements to make emergency calls.
To make a WhatsApp call, simply open the chat with the person you want to call and tap Call in the top right corner.
When you receive a call and WhatsApp is closed, any call notification that you may receive will depend on your notification settings (iPhone Settings > Notifications > WhatsApp).
When you are in WhatsApp, regardless of your notification settings, you will always see a WhatsApp call screen. You will then have the option to accept, decline, decline with a quick message, or decline and set reminder.
For more information about notifications, read this FAQ article.