IIUC VoLTE and Vo5G is technically a VoIP. It is just billed differently by the carrier and the voice packets might have a special priority. Some carriers used to require a data plan in order to use VoLTE, although calls were not using any quota.
When you can receive calls despite having your data disabled, one of two things might happen:
a. You are switched to older network (3G or 2G). Not sure if this switch happens when the call is ringing, or after answering the call.
b. Maybe data transfer restricted to the carrier's VoIP is enabled.
Anyway, I think that the suggestion is risky: Even if it disables calling over VoLTE/Vo5G, it probably will not disable 3G/2G calls. Moreover, while VoLTE (and probably WoWiFi) has a mechanism that can downgrade ongoing call to 2G/3G if it improves call quality, there is no mechanism to upgrade call from 2G (not sure about 3G) back to VoLTE/VoWiFi.
If you really want to call without roaming, you can use pure VoIP (either apps like Signal or generic VoIP providers that give you a phone number). In my experience, it works well even for data roaming, which is essentially a VPN to another country and adds some latency. (It might be easier to get a cheap data roaming than cheap call/SMS roaming, as you aren't tied with your phone number or country. You can buy an eSIM from any carrier over the world and get prices like few $ per GB with roaming.)