@Black Mamba
Hello, thanks for your reply.
Yes, indeed, it requires mobile data in general both in home network and in roaming. And, most probably, you're completely right that many people don't use data roaming (even though in most plans it's included).
On the other hand, as I'm also using CellMapper to collect signal level everywhere I go, I'm using my phone in NR/LTE mode, so it doesn't switch to 3G. Hence, my question. I am able, obviously to use VoLTE in home network, so my phone seems to be correctly configured.
If you check out Calculateur des tarifs d'appels à l'étranger | Swisscom, Germany is probably the only neighboring country where VoLTE in roaming is active.
And to reply to your question, I'm using Samsung Galaxy S22 Ultra 5G Exynos (CH) version. But, as I said above, it seems to be configured correctly, as I am using VoLTE when in Swisscom network