Came across this today: https://hackaday.io/project/19035-zerophone-a-raspberry-pi-smartphone/
Interesting…
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Running Linux with little in the way of power saving… battery won’t last long 
I made something similar, albeit Arduino based — so no “apps” as such, but way lower standby power — a couple of years ago.
https://www.rs-online.com/designspark/building-a-diy-cellphone