Most Xiaomi / Redmi / POCO phones’ Voice over WiFi (VoWiFi) settings are hidden.

I tried the short code \*#\*#869434#\*#\* on the default phone app, but no toast message appeared.

I realised that all short codes are blocked by the default phone app.

The solution is to install another dialer app (i.e. Dialer by GriceMobile) and set it as the default phone app. Then enter the short code, and you would see the hidden VoWiFi settings. You can then remove the new dialer app.

Hope this helps!