Unfortunately, I have to confirm this. Exactly the same for me. I'm only an indirect Sunrise customer because my provider Quickline uses the Sunrise network. I also raised a ticket about this a good month ago. I'm still waiting for an answer or solution today. In my case, it's not the cellar, but an obviously weak reception at my place of residence in combination with a glass/concrete building, which further weakens the signal. I only have some reception in my flat with Salt. WLAN (Internet) works perfectly, but WiFi call does not. This is also a no-go for me, especially in the year 2025. In this case, Digitec should at best include a note about this problem in the functional description of this device so that other buyers don't run into misery. For me, there are only two options in this case: Change provider or change device. It's very uncool that buyers of more durable devices are being penalised.

There are no problems on the swisscom network...

The device requires an OTA update from Fairphone so that WIFI Calling also works on the Sunrise network. Sunrise does not block WIFI Calling and does not operate a whitelist of devices. Please contact the Fairphone support. They should roll out a firmware that supports WIFI Calling on the Sunrise network.