According to a report in Android Police, the WhatsApp calling feature was activated for some selected users with WhatsApp version 2.11.528 from the Play Store or 2.11.531 from WhatsApp website, who saw the caller icon on their app, could make a call to their friends and pass on the invite. The invite in essence was getting a call from someone who already had the calling feature activated. But this invite-feature has since been disabled by WhatsApp.

WhatsApp hasn't made any official announcements regarding this feature and there's no mention of it in release notes on Google Play either. The voice calling feature rollout appears to have stopped soon after its release.

Apart from voice calling, WhatsApp recently launched a Web version that works surprisingly well. Both these features have been available for some time.

Users, who have activated this feature, can still make calls to their friends. WhatsApp has merely put a hold on the missed call invitation feature. Apps like Hike Messenger, WeChat, Line, Viber etc already have voice calling feature, which allow users to free calling as well.