WhatsApp calling will be available only when the developers release an update to support this function. When this happens, you will see a phone icon in the conversion opened. Tap that icon, but make sure to have the Internet connection since it's using standard VOIP to place calls over the Internet for users who signed up to WhatsApp.
To update the application, go to Store, then press the three dot menu/hamburger menu and access Settings. Tap Check for updates and let it update.
Note: An Internet connection is necessary in order to complete this procedure.