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.
I don't think you can call anything with GSMA IP in it "open". Same goes for systems that won't run without their closed binary blobs loaded at boot or runtime to enable hardware.
So far, there has only been 3G-ish experimental SDR-based true open hardware, but it's the size of a shoebox, not a phone.
The Librem 5 is not completely open -- neither the hardware nor software. https://puri.sm/faq/
The Librem 5 is the best effort to this date, but the wifi, bluetooth, and baseband software are proprietary, as well as much of the silicon, and part of the boot chain.
There simply aren't any open source options that exist for these components, and due to regulatory constraints, it's unlikely that an open source baseband will exist any time soon.
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.