WhatsApp provides free calling worldwide. The app uses the phone's Internet connection to make calls; additional charges may apply based on carrier data plans. As of April 2015, WhatsApp calling is only officially available on Android.
To activate WhatsApp calling on Android, first update WhatsApp to version 2.11.561 or later. After receiving a call from a user with the feature already enabled, WhatsApp calling activates on your device. Downloading version 2.12.19 of the app (available on WhatsApp.com) and enabling unknown sources installs the calling feature automatically.
Install WhatsApp on iOS by installing a beta version of the app on a jailbroken device.