After all those posts I'm not sure what you're really after.
As some of us said, you can buy some LTE module and design a basic cell phone (but still 4G) around this, some MCU, LCD display and a few buttons. Ready-to-use LTE modules are not exactly cheap, but there's really no alternative. Designing an LTE front-end directly yourself is a dead-end unless you're very experienced and have an enormous amount of cash to waste. So all in all, with a module, for about $100 of parts, and a number of hours of software coding, you could get there. I think there are already such projects out there that you could take a look at. If all you want is make and receive calls, that's doable. Now as I said, whether you'd actually be allowed to use this, I don't really know, but if you don't intend on selling it, it's for your personal use only, and you use the LTE module as documented, with a compatible antenna and clean power supplies, that should be OK.
Could be a fun project. But other than this, really I can't see why you wouldn't just buy a second-hand old mobile phone with no Android/iOS/Windows Mobile. Something just 2G for instance. I have no crystal ball of course, but I'm pretty sure those are still going to work on most networks for many years.
If you want something more modern, as I think others have already suggested, just select a phone that is supported by open-source initiatives, there are quite a few!
There are Android-based OSs with most nasty stuff either not included or disabled. There also are Linux-based OSs now. Those are safe bets for what seems to be what you want.
So here, you have 3 options. Make your own with a ready-to-use module. Or buy an old phone and use it until no network supports it. Or buy a newer phone and install an open-source OS on it.
Here a list of alternative OSs:
https://itsfoss.com/open-source-alternatives-android/