ZeroPhone is an open-source smartphone that can be assembled for 50$ in parts. It is Linux-powered, with UI software written in Python, allowing it it to be easily modifiable - and it doesn't prohibit you from changing the way it works.

https://wiki.zerophone.org/images/d/d8/ZeroPhone_Gama_version.jpg

https://wiki.zerophone.org/index.php/Main_Page
https://wiki.zerophone.org/Starting_with_ZeroPhone
http://zerophone.org/

https://hackaday.io/project/19035-zerophone-a-raspberry-pi-smartphone