At home, I use VoLTE and the sound is almost impeccable, very high quality, but in the places I roam to, what I get is FM quality 3G sound.
It's not that cellular network is incapable of that sound quality, but I don't get to experience it except my home country. Interesting, indeed.
3G networks in many European countries were shut off in 2022-2024. The few remaining ones will go too over the next couple of years.
VoLTE is 5G, common throughout Europe. However the handset manufacturer may need to qualify each handset model with local carriers before they will connect using VoLTE. As I understand the situation, Google for instance has only qualified Pixel phones for 5G in 19 of 170-odd countries. So 5G features like VoLTE may not be available in all countries. This is very handset/country/carrier-dependent.
Technically, on 5G you have "VoNR"[0], where VoLTE is over 4G.
1. it takes considerable work on my part to understand it on a cell phone
2. it's much easier on POTS
3. it's not a problem on VOIP
4. no issues in person
With all the amazing advances in cell phones, the voice quality of cellular is stuck in the 90's.