Welcome to the community forum.
(I corrected the topic title, you can also open topics in German here, by the way 
.)
Does your mobile plan with your carrier support WiFi calling?
My carrier supports it, but my plan (prepaid) doesn’t.
I got to this setting once via a notification, but it seems to have been a glitch, since then it’s gone for me, too.
It works for users with proper support for it, apparently.