Update 2 (26 March 2015): WhatsApp voice calling invites are open again since 25 March and can be unlocked by installing the latest version of WhatsApp 2.12.14 and then getting an invite from someone who has already activated the service on their smartphone.
Update 1 (12 March 2015): Folks at Android Police have reconfirmed that the WhatsApp voice call invites are once again live for the past few hours after a brief close down from 7 March to 10 March. Interested users are encouraged to grab the latest version of WhatsApp 2.11.561 and get an invite from one of their friends who have access to this service.
It is not known how long this window will remain active and hence users are advised to make most of the available opportunity.
The WhatsApp team has once again activated the calling feature in its servers, and the function has reportedly been confirmed to work for those running the latest App version 2.11.528 from the Play Store, or 2.11.531 from the WhatsApp's website, or 2.11.552 from the APKmirror.
According to Android Police, a few WhatsApp users have recently confirmed via e-mail about their success with activating the App's call service in the past few hours, just after receiving an invite from other users of the call feature.
Folks at Android Police have also confirmed that they have successfully unlocked this feature after receiving a call from one of their close associates via WhatsApp.
Although the call feature is well integrated in WhatsApp, it requires a trigger to get activated and this can be done by receiving a call from someone who is already using the feature.
Consequently, once you receive a WhatsApp call from another person, you should notice that the default UI on your WhatsApp window will change to a three-tab layout with calls, chats and contacts. It is now possible to view incoming, outgoing and missed calls with accurate timing in a dedicated tab.
Ongoing calls do show up in the notification panel until you disconnect the call and missed calls leave a notification in the call logs. When on a call, you can choose to activate the loudspeaker or mute the microphone.
All text chats associated with specific contacts will show a call button in the action bar, next to the attach icon and the menu for the user's convenience.
There is a downside to this feature though, as the call button now defaults to a WhatsApp call instead of linking to a network call, and this means you need to go through the regular phone app to make network calls.
NOTE:Although the WhatsApp calling feature has been tested to work on previous versions of WhatsApp like 2.11. 528 and 2.11.531, it is ascertained that users who are running the latest WhatsApp 2.11.552 will have the best chance of receiving an invite and unlocking the call feature.