An Android, that wasn't even Google Android, and may even look like candy bar.
All it would do would be:
1. Phone
2. Contacts
3. WhatsApp
4. Brave Browser (or Firefox Focus)
Perhaps it would have a camera, but only sufficient for WhatsApp.
Perhaps GPS/Maps would be built in, but only accessible for WhatsApp "share my location".
The browser would open links and allow browsing, but would not integrate too deeply and would focus on security and battery life where updates may not be applied and signal is sometimes terrible.
Just the simplest phone, dirt cheap, durable, capable of communication in an age of WhatsApp.
It would also be a near perfect travel/emergency phone for people in the West, and would likely have a huge market amongst outdoor types.