The Whatsapp Android application has been updated with a new option, the Android application now allows the Whatsapp users to perform phone calls via the client. The Phone calls which are performed via the application use the VOIP technology which uses the internet to create phone requests.
The phone requests are in order transmitted to the designated contact person his client, it is important that the user you are trying to contact also has the client installed on his Android device.
This option is currently only available for Android users, but the community expects that the iPhone update version will be available soon.
I strongly recommend you to use WiFi connections to perform Whatsapp phone requests, the WiFi solution provides the most stable connection for phone conversations via WhatsApp.