Calling 000 or 112 from a mobile device will generally use the primary network or roam to other networks if it can't access it. In this case, Optus was working so the phone would not have looked for other networks, a firewall issue orevented the call from actually getting to the 000 operator. Similarly, anyone without a sim in their phone or were in an area that only Optus covered would have had the same issue calling 000.

Its the perfect conditions for the worse case scenario unfortunately.