On the hardware side, there is a project "Free Calypso" to produce a completely libre (software, firmware, baseband, & hardware) "dumbphone" using the Calypso chipset.

Initially looking to reuse old phones with the Calypso chipsets, the project is now working on producing their own. Design files are completed; funding for the dev boards is about 66% complete.

https://www.freecalypso.org/fcdev3b.html

Mailing list is fairly active too.