WhatsApp Messenger has been updated to version 2.12.10 (released 31 March 2015) on the Tizen Store, and brings with it the ability to make WhatsApp voice calls. This feature went live for Android users yesterday and it's great to see Tizen Samsung Z1 users also getting access this so fast. After the update users are now presented with three Tabs for Calls, Contacts, and Contacts.
You should get a notification to update your WhatsApp application, but if you don't then do the following to manually check and update:
Users in both India and Bangladesh (the countries that the Z1 is on-sale) have confirmed the can access the update. How are you finding the Calling feature? Let us know in the comments.