This is not true of all carriers, only those that rip off their subscribers. None of my 3 providers do this. My Portuguese and German providers will work outside the home country withoutroaming being activated and they charge as if you are in your home country. A mobile data connection will however not allow WiFi calling as a mobile network is, well, not WiFi.

Having said that, if your sim allows free roaming as mine do, then you may not find that WiFi calling is any cheaper if outside your home country and you makea local call in your destination country as this will count as an international call.

Where you are correct, however, is that this is a minefield and many mobile providers have perfected the ripoff. Buyer beware.