Are there even roaming costs at all when calling over wifi? Would make sense to me if there weren't, since you're not using the mobile infrastructure you would be paying for.
Then again I could also easily see telcos charging roaming anyway, just because they can.
Calls can freely be handed over between VoLTE and VoWiFi, depending on signal quality, and that often happens without the user's knowledge or explicit consent.
This means that "no roaming costs over WiFi" is a very dicy proposition, as a carrier either needs to restrict handovers (and I don't even know if that's allowed by the spec, not to mention the implications of dropping calls when going out of WiFi Range), cover the costs themselves, or move them onto the unsuspecting user if a handover happens.
There's also a technical hurdle of the telco can't know where you're coming from over the Internet. The terminate your end of the vowifi call and from there they only charge for the connection as though it originated in their network, which is all they know for sure (that is their SIM and your account that's authenticated from somewhere in the world)
Well, I know at least one provider that looks at your IP's country registration and will block you if they believe it's not domestic: Vodafone Germany. I wouldn't wish their service upon my worst enemies.
Then again I could also easily see telcos charging roaming anyway, just because they can.