My phone phoned! It was actually easier than I expected. I just had to launch the lomiri-dialer-app (which is pre-installed, since this is ubuntu touch). I though it only supported Mir, but it actually works just fine on Xorg.
If you want a Linux phone, buy a PinePhone, install PostmarketOS with Phosh and continue enjoying your life. In the meantime, I will continue trying to receive phone calls, SMS messages, notifications and the “_” character
It would be difficult to provide a guide, since this phone’s configuration is split between two of my pseudonyms.