I'm with Wingo and it works for me, including the new Vo)) Icon.

For me, from top to bottom, it says : Roaming, Data usage by apps, Activate VoLTE, Preferred network type, Calls via WLAN, Select network automatically, Network Swisscom (cannot be changed) and Access points. In any case, your service provider and SIM card must be compatible. LTE network must be receivable. On the Sunrise page, I can't find a Sony phone in the list of supported devices. Definitely install Android 9 (if not already done). My exact model name : G8341. My setting is also on 4G preferred. If it doesn't work, Sunrise doesn't support VoLTE on your device.

Hii. I don't know about the CH market. The mobile phone does not support VoLTE/VoIP when calling Swisscom, I don't know because I'm with Sunrise and not Swisscom.

I assume that a device bought at Digitec is for the European market. It has VoLTE and "calls via WLAN".