In the Salt network (and probably in others too):
there is an obvious but easy-to-overlook side-effect of Wi-Fi calling while abroad. In Salt there is a CHF5,- per month "Go Europe" roaming cost-reduction option, where calling and receiving calls in some countries in Europe costs CHF0.20 per min.
However, when using Wi-Fi calling one is considered to be in Switzerland, and the regular rate applies, i.e. outgoing calls to foreign numbers are CHF0.79 (instead of the CHF0.20).

Also, if you have access to a Wifi, why would you not use something like a Skype call (to mobile/landline) which is much cheaper than that ?

Skype uses a masked phone number, and in Poland many people and businesses block such incoming calls.

why is wifi calling not free??

Because you use wi-fi, but your recipient highly likely doesn't.

and why am I paying for his service?

You aren't.

You are paying for your provider to connect to their provider.

You can setup caller id in Skype https://www.skype.com/en/features/ca...dentification/