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, according to a report.
(Also see: How to Backup WhatsApp Messages)
WhatsApp appears to be rolling out this feature in short phases with Android being the first platform to get it. WhatsApp for iPhone still doesn't have this feature even though a voice calling button appeared in the app some time ago.
After establishing itself as the world's most popular messaging app with over 700 million monthly active users, WhatsApp seems to be playing catch up by adding features available in competing messaging apps. Apart from voice calling, WhatsApp recently launched a Web versionthat works surprisingly well. Both these features have been available for some time with competing messaging apps such as iMessage (calling via FaceTime), Line, Viber, among others.