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.

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.