Still testing.
-Fresh install of Fairphone 5's dev r43 build.
-Installed ofono plugin with the previously stated instructions and changed qti.conf.
Phone still shows LTE + VoLTE icons. If I call my voicemail it still shows LTE + VoLTE, so I assume no 2G calling. During my voicemail calls, I cannot register any button press such as entering my voicemail PIN. It shows the button press on my screen and makes the sound, but the call does not register input on their end, thus I could not listen to my voicemails.
Also I was unable to hangup, I had to reboot the phone. Opening the dialer app without reboot showed it immediately "calling" the last number without telling it too, but it never actually calls. Reboot is what fixes this.
"gdbus call --system --dest org.ofono --object-path /ril_0 --method org.ofono.IpMultimediaSystem.GetProperties"
shows:
({'Registered': <true>, 'Registration': <'auto'>, 'VoiceCapable': <true>, 'SmsCapable': <false>},)