Had an interesting issue while overseas where my wifes mobile account was charged the daily roaming charge on the days an SMS text was sent via Wifi Calling.

The phone was set to flight mode before we left NZ and then Wifi and Bluetooth were switched on. Got to our resort in Fiji, connected to the wifi and the Wifi Calling icon was displayed on the phone. We were there 8 days and on 4 days my wife received texts and replied. These were the days the roaming charges matched.

2 Degrees did reverse the charges, but the helpdesk agent tried to tell us that SMS definitely doesn't work over wifi calling, and that sending an SMS somehow overrode the flight mode setting and sent through the local network.
I saw both flight mode being set before we left and unset when we got back, so definitely know it was on. Also if it was accidently turned off I would expect to receive the Welcome to Country SMS texts you normally get. The phone is a NZ purchased Oppo A78 5G which had prepaid 2 degrees sim card in the box (however is used on-account). 
It almost seems like the phone has somehow partially connected to the Fiji network, to trigger the roaming, and as we were planning on using wifi calling later in the year on another overseas trip would like to ensure this doesn't occur again.