Then that means the phone isn't using VoLTE.
Telenor was the first operator to have VoLTE a few years back... On one device or something (AFAIR), but it's time these slugs launch it across the spectrum.
Interestingly, no operator supports Apple nor Pixel devices, which are the most standardized phones (in the sense that there are only a few variations in each model across the whole world) so they're more alike "global phones". Samsung makes phones for different regions, so do Oppo, Huawei and the rest.
Yet, these are not supported.
Welcome to Pakistan, where Jazz just told me to change my handset because the network lady told me my number is invalid because I was dialing 051-1218, and 15 wouldn't connect (it happened recently and I'm not going to test it now).
I mean - change the handset for a "number not recognized" error from the network end?