Ask HN: How can I make a custom dumb phone?

I'm looking for any tips or pointers to resources I could use for designing a custom flip/slider cell phone. I want it to be able to make calls, exchange texts, and (maybe) run simple apps that access the internet.

My basic motivation is: I much prefer small phones that have actual buttons over the ubiquitous large rectangles with touch screens of today's world. Unfortunately, phones like that that are available today either are a) too old and use outdated networks, b) new but seem like they have awfulness engineered into the UI as a feature to push people towards smartphones. None of them seem even remotely hackable, because they're all proprietary. The new open phone OSes (like pinephone) are encouraging, but everything still seems to be pretty firmly entrenched in the smartphone paradigm, and don't run on semi-dumb phones.

I've hacked a bit on arduinos and raspberry pis, so I could figure out how to wire up a pi with a 4G modem with a battery and a screen. The problem is: it would be huge and clunky, and I haven't found a 'maker' friendly phone keyboard or slider/flipper mechanism.

So.. my question for you all is: - how would I begin prototyping the hardware involved? -- OR -- - how would I go about taking a relatively new flip/slider phone and making it run my own OS instead of whatever proprietary code it came with? -- OR -- any other suggestions?

Thanks!