Here are steps I've just followed:
- Flash Android 11 stock image from fairphone website
- Start Android 11, configure phone
- In mobile settings, uncheck VoLTE and check it back
- Same for wifi calling (uncheck then check)
- Make a call and check it's working well with Android. My operator has disabled 3G, so this check ensures me the VoLTE is well working on Android
Then, download UBports Installer 0.11.2 and start the process to install Ubuntu Touch 24.04-1.0 stable channel wipping user data.
Phone seems connected to 4G: the network status icon has letter ยซย Lย ยป. Although voice phone call did not work.
I tried to call my Ubuntu Touch device with another Android phone and I didn't received the call on the Ubuntu Touch one.
I tried to run the gdbus command and I got same result than you (neither registered, nor voice and sms capabilities).
On a new test, I've started journalctl -f and I attach the result here. Let me know if more logs are needed.
voice_call.log
Note that before doing the call, I disconnected wifi to be sure to use VoLTE and not VoWifi.