Comment by jeroenhd

VoLTE roaming isn't quite as easy as "log in to the network and connect to the local SIP server". There's a lot of interplay with the home carrier: https://moniem-tech.com/2024/01/01/what-is-volte-roaming/

It's perfectly possible for VoLTE not to work in regions where no carrier provisioning information is available while foreign SIM cards work fine.

In theory a phone can just be provisioned by the network to use VoLTE, but in practice the spec allows for all kinds of incompatible configurations. Carriers and phone manufacturers won't just apply an untested configuration, and for good reason. Software upgrades have broken telecommunications from iPhones to Androids, sometimes edge cases such as calling 111/112/911/999 turn out not to work.

Falling back to 3G or even 2G on unknown networks in unsupported markets will get you voice calls, at least for the coming years.