OK, tried more things, and finally “solved” it: I just received an update for the native phone app, and in there now the option appears under
- Three dots
- Settings
- Calling Accounts (guessed - German: Anrufkonten)
- Tap on the SIM-card
- WiFi calling (guessed - German: WLAN telefonie)
Finally… However, in the normal settings app the setting is still missing…