I even got a new sim card recently and I can't activate WiFi calling. Too bad... I'll ask Salt about it.

What I can say is that it often depends more on the sim card than on the mobile phone.
(I had WiFi calling on my mobile phone, changed within the same provider and poof, WiFi calling was gone on the same device. The setting also no longer appeared in the mobile phone settings. No one could explain it to me; they simply told me to order a new SIM card... I didn't do that...
(Maybe it's some kind of lottery?)