We'll assume you have put a phone SIM into it, and not a broadband SIM.
It's tempting to blame the SIM outright, but is the phone showing 4G? Is it attempting to do a 4G volte call, that fails? Can you lock it to 3G in the whatever setting?
What do I know?
You in WA or Perth? Saw 2 other posts just like this, I'm assuming some sort of outage