I tested all options now on my provider. (Your experience may vary due to different bands available at your provider.)
I realized I have the VoLTE ringtone bug on 4G VoLTE, it is somewhat annoying but not really a huge deal. Switching VoLTE off solves the issue for now as seen below. I still think that this is the best current option for SFOS and well usable as a daily driver.
- Prefer 2G: goes to 2.5G at my provider and calls are ok. Outgoing ringtone OK
- Prefer 3G: behaves same as above
- Prefer 4G with 4G calling (VoLTE) OFF: Mobile internet OK after calls, Calls OK → falls back on 2G as intended, Outgoing ringtone OK
- Prefer 4G with 4G calling (VoLTE) ON: Mobile internet OK after calls, Calls OK → calls made on 4G as intended, Outgoing ringtone and feedback sounds Not available.
- Prefer 5G: Behaves as Preffer 4G above. I Suppose there is no 5G available in my area or would need some additional settings.
I may go abroad in some weeks and may report back in case I see different patterns with other providers.