WhatsApp can now be downloaded to all Kai OS powered phones. Previously, it was available on select models like the JioPhone and the Nokia 8110 4G (which got it with an update).
The app has been optimized to run on phones with 256MB or 512MB of RAM. Here's the official list of all Kai phones – currently, there are 12 models, 8 of which have 512MB of RAM, the rest have 256MB.
WhatsApp for Kai OS supports both calls and messaging, complete with end-to-end encryption.
Anyway, as many people have moved away from SMS to instant messaging apps, buying a feature phone meant getting cut off from the conversation. Not anymore, WhatsApp is used by 1.5 billion people and now even more can join in.
As for now, 100 million phones with Kai OS have shipped to over 100 countries. As of Q3 this year, most Kai OS phones should come with WhatsApp pre-installed (the Facebook app is already available in most regions).
KaiOS Mobile software