Arsenijs build a Pi-powered open-source mobile phone (that you can assemble for 50$ in parts).
Currently, it costs about 50$ in parts, and all the parts are available on eBay. No BGA or other difficultly solderable ICs are used (with the obvious exception of Pi Zero). User interface is written using Python, and there’s a phone-tailored UI framework in the works (so far, it uses pyLCI for interfacing). However, even current state of it is further that other projects have come.
ZeroPhone – a Raspberry Pi smartphone – [Link]