Voice calls on WhatsApp have started to appear on smart phones with KaiOS. In particular, the corresponding update has become available for Jio Phone, but should appear on other devices with this operating system.
Calls to WhatsApp work as part of VoIP technology, of course, requiring an Internet connection.
As a reminder, KaiOS is an operating system for conventional push-button phones. It can’t compete with iOS or Android, but it still offers many modern features and programs not available on other regular phones. Besides WhatsApp, there are YouTube, Facebook, Google Duo, games and many other applications.