“WiFi calling” is used in countries where the mobile signal is often crap at home but people can get internet at home via some other means. (The technology used for the home to connect to the internet does not matter. In theory it could even be mobile broadband. )
The phone connects to the WiFi (easy) then connects to the internet (presumed to be easy) and then connects to the mobile service provider for the mobile phone but over the internet.
Then the mobile service provider allows the phone to make calls as if being made over the mobile network e.g. billed normally, surveilled normally, …
And, yes, it only works if the mobile service provider supports it and allows it - and if the phone supports it.
It is quite different in implementation and set up from VoIP although at the end of the day it provides similar functionality.
Of course there is nothing magic about “at home” in the first paragraph above. It can in theory be used anywhere the mobile phone can get to the internet via WiFi.
To complicate matters, on a cool phone like the Librem 5 where you can plug an ethernet dongle into the bottom of the phone, there is even nothing magic about the “WiFi” in “WiFi calling”. If “WiFi calling” works at all then you can presumably do “gigabit ethernet calling”.