Using existing phones to run Linux on them (preferably mainline) takes away the problem of creating a housing and possibly hinges or sliding mechanisms for the hardware keyboard. Those mechanical problems are actually quite hard to solve as a “hobbyist” on a non-vendor scale.
Phones that (somewhat) run Linux
This document is still WIP, so the list is probably not complete yet
While this clamshell mobile phone by Planet Computers is marketed as multiboot device capable of running Linux, the kernel used at least in the community-built(?) Debian image is the ancient 3.18 kernel hacked together for their Android image, with some additional patches. So mainline Linux has still way to go on this device, although there seem to be some spread out efforts.
I collected more resources on this topic here: gemini.irimi.one
Motorola Droid 4
The basics for this phone seem to be available for mainline Linux already. That is, if you don’t intend to make actual phone calls or take photos with it. Europeans might know this device as “Motorola Milestone 4”.
Here’s a list of resources:
Although the hardware of this device is already fairly outdated, there are efforts to get mainline Linux running on these phones. In fact, you are already able to install Debian-based Linux-distributions on them, with some drivers still missing if I recall correctly.
Here is a list of resources: