Hi there,
It looks like VoLTE is not yet unlocked on this phone. If you are a tech enthusiast, you might want to check this thread on XDA-developers out. It helps to unlock VoWiFi, but require getting root access to the phone.
xda-developers.com/enable-v...
I'd personally prefer to wait until the function becomes available to the mainstream users.
Best regards,
Greg

Unfortunately, I can't help you any further. Please ask Swisscom directly.

Hello. With the latest update, VoLTE and WiFi calling has been enabled. At least with Sunrise, but I suppose it should work with other providers too.