This is third-hand info, but apparently the Optus situation was heaps cooked.

Every one of their devices had been deregistered and needed to essentially “register” with their servers, i.e., a tower.

A similar thing happened with Telstra a while ago that impacted only Vic and Tas, but it took them ages because they had to take a bunch of towers offline and slowly bring them on so that all the devices constantly trying to re-register didn’t brute-force them and take things offline again

Multiply that by a few hundred thousand/million devices and that’s what Optus was dealing with… Oof