“Rule number one, never carry anything you don’t control.” (See Andor.)
How would one follow this advice in this world?
One would need only computers with a 100% open source stack. Including hardware.
Imagine starting with a mass-produced phone-sized touchscreen and battery that contained zero processors. You would then build a computer that plugs into that and screws onto the back. That would be your smartphone. You control all processors.
Imagine a RaspberryPi-Zero-style RISC-V computer that was 100% open on every integrated circuit. you could use that as the core of your fully-secure smartphone.
You would need to reverse-engineer a bunch of currently-opaque hardware that's hidden in special-purpose chips. E.g. cell network antenna drivers.
I would also build the hardware such that a led lights up whenever a camera or microphone is enabled, without the ability to change that in software.
And the software stack would need to be controlled by a non-profit with a long history of respecting users. Maybe Debian? I'd love just to have a phone that runs Debian!
It looks like there are several raspberryPi-based phone designs out there. My main problem with that is that it uses processors made by Broadcom Inc. These system-on-a-chip processors are wonderful technology, but the hardware is closed-source, and has closed-source firmware and graphics drivers.