WhatsApp’s Free Voice Calling feature has been rolled out to iOS users. The new version of the messenger is now live in the iOS App Store with support to calling. The feature allows you to call your friends and family anywhere in the world for free as long as you have a Wi-Fi network or a working data plan on your registered number. WhatsApp calls use your phone’s Internet connection rather than your cellular plan’s voice minutes (Data charges may apply) The feature, so far, is not actually available for all iOS users, but rather will be available in the next several weeks.
Source: https://thehackernews.com/2015/04/whatsapp-ios-voice-calling.html