Question about Xiaomi Wed 9 SE

Both Swisscom and Sunrise maintain a whitelist for WiFi calling. This means that they only add individual devices. Usually only those that they sell themselves. It is therefore irrelevant whether the device supports the function. This is very annoying, not customer-friendly and extremely restricts the choice of devices.

I have checked with Digitec Connect Support over a few weeks now, final answer from Sunrise is that the device is not supported by them (but that's not because of the device!).
(By the way, I noticed that Sunrise doesn't support quite a few devices, so Digitec might be well advised to change providers...).

Have you activated it on the phone itself? It is usually not on by default.
Via the dialer:
VoLTE *#*#86583#*#*
VoWIFI *#*#869434#*#*