Schiller
It probably depends on either sim config or os config. For example, on my Samsung, wifi calling often works when there is a good signal but its not always like that - sometimes it does, sometimes not, always automatically.
On Pixel 7, it only works when there is no signal or its in airplane mode.
Maybe it will be possible to control it by messing with IMS settings ##0702## but I personally think this is not a feasible goal.