While this may seem terribly ungainly, Nokia had a debian-based, smartphone-sized touchscreen tablet 18 months before the iPhone was released. https://en.m.wikipedia.org/wiki/Nokia_770_Internet_Tablet

This isn’t the slam dunk that people think it is, unfortunately.

You see, in addition to controlling the cellular radio and all of those details, the bass band processor also does real time noise cancellation and a variety of other call quality functions that you would immediately miss if they were not there.

That processor is actually doing a lot of different things and is difficult to remove from a phone and maintain what most people would consider an acceptable user experience.

I guess, but VOIP and wifi calling works.

I agree. That's the best way to go. But it is not mutually exclusive with never being able to own your smartphone.

My point was more that if you can have a fully free PC except for the cellphone modem, then you can also have a fully free cellphone except for the modem. There's nothing about the rest of the cellphone that's any different from any other computing device.

As another commenter pointed out - the Pinephone is device attempting to do that.