Popular messaging platform, WhatsApp is currently testing voice calling functionality for the Windows Phone version of the app. Voice calling functionality has been available for Android BlackBerry and iOS users for a while now.
The new feature is enabled on WhatsApp version 2.12.238 beta build, but you have to be on the WhatsApp beta testing for Windows Phone program to try it out. You can find out how to join the beta testing program here.
There is currently no info as to when this feature will be available for all Windows Phone users. Hopefully, that will be soon.