Says the OS is proprietary but the description refers to "Android and Worker Productivity Applications". No idea what those are (and don't care at this point): https://www.verizon.com/basic-phones/sonim-xp5s/#specsHeadingAre there any non smart VoLTE phone out there? If you go below VoLTE then your network may no longer support you after 1 year or so.
Or this one: https://www.verizon.com/basic-phones/dura-xv-extreme/#featuresHeading. No mention of the OS.
Or this one: https://www.verizon.com/basic-phones/alcatel-go-flip-v/. The UI on this one looks pretty nice, in fact. Says the OS is proprietary Alcatel.