Telstra has fessed up to the cause of last week's nationwide mobile outage, and it's a tale as old as time - or at least as old as 2006. In a written submission ahead of CEO Vicki Brady's appearance at a Senate inquiry on Friday, the company revealed that a software update wasn't applied to a key time-keeping server, and maintenance workers were kept in the dark about a design change that affected how it would reset.

Here's the gist: Telstra has three Network Time Protocol (NTP) servers, in Melbourne, Sydney and Perth. During routine maintenance, the Melbourne server was shut down and restarted. But due to an "underlying software configuration," it rebooted with the wrong date - specifically, the year 2006. Over the next few hours, that incorrect date "rippled slowly across the network," causing authentication certificates in other servers to become invalid. Customers were intermittently unable to connect, make calls, or use data.

Telstra insists it has redundancy - the Sydney and Perth servers worked fine as backups. The problem wasn't a lack of redundancy, but that the Melbourne server, once back online, fed bad data to downstream systems that rely on timing for security and authentication. "The failure mode here was not inherently related to hardware, levels of redundancy, or the architecture of our network," the company said.

Telstra also acknowledged that an intentional design change to fix an earlier fault wasn't properly documented, so maintenance workers were unaware of how the device would behave. On top of that, a software update that could have prevented the whole mess hadn't been applied. The company is taking "full accountability," admitting, "If maintenance work can trigger this kind of outage, it suggests our controls were not good enough."

During the outage, 58,835 calls to triple-zero successfully connected, but 604 experienced errors. The triple-zero platform wasn't affected because it doesn't use those NTP servers. Fixed-line callers on the NBN were also fine.

On Friday, Brady and other execs will face a Senate inquiry originally established after last year's Optus outage. Committee chair Senator Sarah Hanson-Young said the goal is to "get to the truth" and ensure Australians aren't left vulnerable by such failures. Maybe they'll also ask why a network can't tell what year it is.