Really tried to use the OpenMoko and almost every other open source phone that came along for so long and never had any success.
I found SailfishOS UI to be pretty bizarre and for using compiled software (Qt) the UI was oddly laggy. Basic things like attaching photos when sending an MMS were bizarre, you had to do it one at a time from the gallery app, etc.
Ubuntu Touch has a pretty reasonable UI (aside from having a start-bar like thing that you swipe in and out) but they do NOT support VoLTE at the moment which means you can't use it as a phone! Performance on Ubuntu Touch was also pretty bad even on decent hardware and while it's improved on Pixel-era devices you'd think all UI interactions would be instaneous.
Both projects seem to have made some strane prioritizations. Sailfish seems to have pointless re-invented their own UI paradigms, their own browser, etc and Ubuntu Touch doesn't seem to care the devices can't be used as phones given lack of VoLTE but they have Snaps and their own web browser...
Unfortunately I managed to drop it and now the screen has a dead spot in the bottom left. As that is the spot where the important icons are it means I have a device that works perfectly aside from being entirely useless (a device which resembles its owner perfectly).
VoLTE is hard but JMP.chat can act as a workaround.
Based on this Stackover flow post I guess it CAN be compiled but that depends on the version, open-source, commercial, etc:
https://stackoverflow.com/questions/9448296/is-qml-translate...
JMP.chat exists but honestly that's kind of a crutch or work around if your primary cell phone can't make or receive calls on its own without ANOTHER broker or provider.
One thing I did like about Sailfish OS and to a lesser extent Ubuntu is that it's mostly "standard" Linux under the hood so you can do things like run dnsmasq or something like that and tweak it as needed.
But in this day and age I think the bare minimum for a cell phone is calling, texting (SMS), possibly MMS and GPS/maps that don't need a network or Internet connection. I'm hoping PostmarketOS, Sailfish, Ubuntu Touch, open-source-smart-phone will get there but I'm not holding my breath.