Country code missing when receiving call when roaming

I'm using an Australian phone number with international roaming, currently in Georgia (although this applies elsewhere).

When I receive calls on iPhone the international prefix is missing so the call incorrectly comes up as "Georgia" instead of Australia. The trouble is that it reliably breaks my contacts stored on the phone (as they're incorrectly identified as another country) and makes it nearly impossible to filter out spam calls.

I've tried turning on/off Dial Assist but that seems to only matter for outgoing calls.

Is there any known fix for this?

The following calls, for example, all came from Australia