You will incur International Roaming charges on the calls made & received on your phone, regardless if it's through the cell towers OR on a WiFi connection.
When using WiFi calling, the call will route through the cellular networks as much as possible before switching to internet networks for the "last mile" (between the nearest cell tower & your physical location). Since the call is passing through international boundaries, the carrier has to charge international roaming rates to cover the cost of transmitting the call through the foreign networks.
It's HIGHLY RECOMMENDED to disable WiFi calling when traveling internationally to avoid roaming charges since you can re-enable WiFi manually after engaging Airplane mode. If there's a way for a call to reach your phone while you're outside your country, you will get hit with call roaming charges. Even if you don't answer your phone, you will still get hit for a minute of roaming since the call gets routed to your phone (outside your country) first, then your phone redirects the call to your voicemail box (in your country) when you don't answer. If you're phone is completely off or disconnected from the cellular network (& WiFi calling disabled), the call would go straight to voicemail, avoiding the international roaming charge.
Please be aware that I have not mention any data charges here (until now) because you disabled your cell data (& likely have data roaming disabled as well, which will prevent you phone from using foreign cell towers for data) & WiFi calling doesn't touch cell data AT ALL... so you won't see such charges on your bill.
During my tenure as a customer service representative with a cellular carrier, I provided such advisement (like disabling data roaming, turning off WiFi calling, keeping phone on airplane mode or off as much as possible to minimize unwanted international roaming charges) for those traveling outside of the US as I didn't want my customers to have any nasty surprises with their bills & having to beg for a one-time courtesy re-rate (which are only permitted once every 6 months at the most & a massive pain for the reps to run the figures & clear them with managers as they typically break adjustment limits).