Any one of us here could learn the skills to design a smartphone. It won't necessarily be good, but I remember that years ago, someone made one with a touchscreen hat and GSM hat atop a Raspberry Pi, rubber-banded to a power bank. I'm sure any one of us HN users could do this. And it worked. Quality only goes up from there.

The problem is it won't run any apps, so you'll need to carry this open-source secure phone in addition to your normal phone.

Or use everything via the web browser; but yes, I think apps are the main reason we can't just have a generic Linux phone OS on an open hardware platform