Maybe start with one of these... https://hackaday.io/project/19035-zerophone-a-raspberry-pi-smartphone/log/51839-project-description-and-frequently-asked-quest

Maybe start with one of these... https://hackaday.io/project/19035-zerophone-a-raspberry-pi-smartphone/log/51839-project-description-and-frequently-asked-questions https://hackaday.io/project/19035-zerophone-a-raspberry-pi-s... https://openalia.wordpress.com/2012/06/09/diy-totally-open-source-arduino-gsm-cell-phone-by-zach-wick/ https://openalia.wordpress.com/2012/06/09/diy-totally-open-s... ...then start deleting code out of it, hardening it, and using memory-safe apps. A nice step on the way to a truly, dumb phone that might take a lot of custom work.