The ZeroPhone is based on the raspberry pi.
https://hackaday.io/project/19035-zerophone-a-raspberry-pi-s...
Reddit AMA: https://www.reddit.com/r/raspberry_pi/comments/5nwmfx/im_mak...
$50. Calling, SMS. Alarm clock, calendar, calculator, phonebook, file browser, web browser and music player. I expect these will be very simple given the screen.