WHATSAPP CALL NOW OFFICIALLY OPEN TO ALL, NO INVITES REQUIRED JUST UPDATE TO V 2.12.19

The WhatsApp calling feature is working. It opens seamlesly, no questions asked, no invites needed, no hacks required. You only need to update the app to version 2.12.19, which you can find hosted on APK Mirror or on WhatsApp's website.

Once you download the apk file and install it (make sure you have Unknown Sources enabled in your phone's Security Settings to be able to do so), you'll open WhatsApp and the app's UI will have switched over to the new one with the Calls functionality enabled.

Over at the AndroidPolice blog, Rita El Khoury (hat off for the tip) has explored this and noted:

1. You can initiate a new call to any contact while chatting with them or browsing the list of Contacts, but if their version of WhatsApp is still not compatible, you'll get a warning. 
2. If you try to call someone who is offline at the moment, you'll hear the phone ring but no one answers and the person will be notified with a missed call the next time they are connected.
3. While on a call, WhatsApp lets you switch to speakerphone mode and mute your microphone should you need that, and a notification shows up with the duration and the option to hang up. But keep in mind that there no longer is an option to place a network call to a contact when you're inside the app, so you will have to remember to head out to your phone's contact list to do so.