Question about Google Pixel 4

Hello all. Can anyone help me regarding the activation of wifi calling on the Pixel 4? I was previously a Swisscom customer, however I had to find out that this provider does not support the Pixel in Wifi-Calling (is unfortunately necessary in my house bunker). After a short research, I have now switched to digitec.connect (provider Sunrise), but the option for wifi calling is nowhere to be found, although this is explicitly advertised. The option to activate it can neither be found in the settings; network & internet nor in the phone app, as is claimed everywhere on the net and also on the Sunrise support page. Digitec customer service has not been able to help me so far, which is why I am now turning to you...