Thanks, depending on the fault, it would either be repaired or replaced. Before that, if not already, I'd suggest a factory reset. If that fails, please head into a store so we can have it looked at
My workmate picked up Galaxy S9 from Optus last week. I can confirm the VoLTE icon is on the status bar and there is no VoLTE toggle option in the Network or Phone setting. The only different is my workmate's model is G960F.
I think it is the Optus ROM that might hide the options. Worth going to a store to confirm, however, I think that might not resolve the issue that you are having. Maybe Optus can OEM unlock for you and load the Global rom instead of the Optus rom.