Whatsapp has today issued an update to version 2.12.1 to enable Voice over Internet Proticol (VOIP) which effectively means free calling over the Internet but app is not as rosy as it seems. 

When making a call to a number abroad on wifi it makes the call however if you do it over your cellular network the call is routed through you normal carrier and obviously fees apply. 

 
Another downside is that if you are jailbroken and use the tweak online notify Whatsapp stops you from launching the app.   

Hopefully the maker of this tweak will be able to update to work on the latest version.