On some flavours of Android (Can only talk for the one I use: GrapheneOS, that is running the latest android to date (16 QPR1)), there is a system setting (Settings > Network and Internet > SIMs > Sunrise > Carrier settings overrides) that supposely allows force-enabling VoLTE. Maybe it can help.
Although, before that, you might just want to check in “Settings > Network and Internet > SIMs > Sunrise” that you have a VoLTE, in which case just turn it on. On my Pixel9Pro, it is the case, so no need for any override shenanigans.