So since posting this, every so often I would test by turning WiFi calling on to see if something happens.
Today for the first time, without wasting my time calling MTN or going to a store again, it finally actually works.
Still the same phone, no software updates since end Apr24, same sim, same...
Sorry, didn't see your reply until today.
I just checked, I don't even have the option to turn VoLTE on...
Now to find someone at MTN who knows it is something on their side. Fun
Enabling WiFi calling on my S22 does nothing, but enabling it on my wife's S10 works fine (which is my old phone, where WiFi calling did not work for me; it also worked for her on her old P30 lite).
We are both on MTN, and use Afrihost for our fibre.