now that we've diy laptop, what about a diy smartphone! too big a challenge? :o :D
1) we dont have a diy laptop
2) the problem is gsm modem. Also formfactor.
what problem is the gsm modem - there are plenty of such modules, the question is what value will have such diy smartphone? it will have one main board, LCD and few plastics, too easy to assembly, no much educational value, with the laptop is another situation - you can make different add-ons inside, change batteries, main board, etc etc to make it lab - Digital Storage Oscilloscope, Logic Analyzer etc etc
with phone the small dimension restrict you on this
look hacker's DIY tablet looks like good thing to think about :)
something like: https://www.crowdsupply.com/knivd/ello-2m but with more powerful processor, running linux and FPGA... :)