The operating system is not the missing part of the puzzle. There are plenty of FOSS operating systems already.

The missing part of the puzzle is that phones are much more than an OS. To get a phone, you need an SOC, baseband, drivers, regulatory approval, manufacturing, etc. Those are the missing parts.

I think it's a pretty futile effort to make a completely open phone. Even the richest companies on the planet outsource much of the above. Pretty much every previous effort to make an open phone has failed to make much of anything open beyond the OS, let alone all of it.