Maybe?
Afaik wifi calling works the same as VoLTE, which afaik is just VoIP, but handled by your mobile provider, and integrated into your normal mobile plan, so it works with your phone number.
When both ends of a call have the service, the call is entirely VoIP, which is how it achieves the much nicer audio quality for these calls.
If you just completely disable the mobile radios (set your phone not to use any kind of connection from the sim settings, while still enabling the sim) and use wifi exclusively, then it should work as if you just have VoIP service?
It’s not. GSM calls are definitely still a thing, and while VoLTE is becoming more and more common, it’s not universal.