That is simply not true. What you get is standards. Standards bodies exist. In this most recent debacle all that is needed is compatible bands and VoLTE to make it compatible.
If these two exist, then it works.
Don’t listen to me, here is a reference. https://www.whistleout.com.au/MobilePhones/Guides/Will-my-phone-work-in-Australia-carrier-network-frequencies
Please teach me