If your "dumbphone" doesn't support VoLTE you'll be frustrated with dropped calls and spotty service everywhere. I know this from experience.
I settled on T-Mobile's only "dumbphone" - the Alcatel GoFlip [1]. It supports LTE, the battery lasts for days and it even has a rudimentary IMAP and CalDAV client.
I tried turning my smartphone into a "dumbphone" and using it less, but I honestly lacked the self-control to put the stupid thing down. Having a kid made me realize how many moments I was pissing away just scrolling.
These things are designed to be addictive, and some people (like myself) can't compete with the entire teams of "engagement engineers" many tech companies employ to exploit your dopamine-reward cycles.
I had forgotten what it was like to be alone with my thoughts. I feel as though I am rediscovering a part of my brain that was suppressed by having a boredom-prevention device at my fingertips all these years. Also other humans now look like phone-zombies everywhere I look. It's kind of dystopian honestly.