I already have the Oneplus 8 pro. It did not work for me. However, I was able to switch it on with these instructions👍.

https://www.theandroidsoul.com/how-to-ena...

OTA Update Heute 01.06.2021: VOLTE / WIFI Calling läuft.

What does it look like now after the Oxygen OS 11 update?

OnePlus is not officially represented in Switzerland and therefore access to functions such as Wi-Fi Calling and 5G, which unfortunately both require country-specific unlocks in the software, will probably continue to be blocked. However, there are instructions on forums.oneplus.com to unlock the functions until the next update. However, this will have to be repeated.

The switch does not appear for me. Even after following the instructions above. Salt.

No, unfortunately not. However, Oneplus has activated the option, so it is up to the providers to activate it. You would have to ask Swisscom/Sunrise/Salt if something is planned. The devices themselves support Wifi calling and VoLTE.