Telstra's boss has revealed a series of embarrassing failures, including incorrect emergency services numbers, prevented Australians from speaking to triple zero call takers for more than an hour.
Vicki Brady on Wednesday detailed what led up to the March 1 outage, saying the issues started at 3.30am when callers' locations and phone numbers - or "calling line identification" - were not coming up on calls.
Telstra needed those details to transfer callers to emergency services, and so had to switch to its backup process whereby operators manually asked callers where they were located before passing them to the correct emergency service.
Telstra managed to successfully transfer 346 calls through this process.
However, another 127 languished in wait because the telco had incorrect emergency service numbers on file.
Telstra had 24 backup phone numbers for state emergency services on file and eight of them were wrong, Ms Brady revealed.
That meant the 127 callers had to wait for an email transfer and call back, while another 21 said they no longer required emergency assistance.
The failures did not end there, though, with Ms Brady noting further delays in call backs because a staff member incorrectly typed in an email address.
"When our team identified an issue with the phone numbers they resorted to email, and were supplied an updated email address for Triple Zero Victoria during the incident," the Telstra boss said in a blog post on Wednesday.
"This address was incorrectly entered into the system. Our team identified the error within 13 minutes, but this did still cause a delay.
"Ensuring we have the right contact numbers for emergency services operators is basic and something we should have gotten right."
Ms Brady described the email error as happening in the "heat of the moment" and said call back delays mostly affected people in Victoria.
An investigation ultimately found the calling line identification issue happened because a triple zero database maxed out, triggering an existing but previously undetected software issue.
Ms Brady was personally overseeing Telstra's work to avoid further failures, including updating instructions to staff to diagnose and address any future issues quicker.
The telco had also increased the capacity of its affected database and was reviewing its approach to triple zero processes.
Within 90 minutes of the incident starting, the service returned to normal, Ms Brady said.
"Let me reinforce that the series of failures that occurred on March 1 are unacceptable," she said.
"Australian public rely on triple zero in their times of greatest need, and we let them down by not being prepared enough for the situation."
She apologised to everyone affected, but in particular, the family of a man who suffered a cardiac arrest and died.
Telstra would roll out a software change to fix the technical issue that led to the triple zero outage by early April, Ms Brady said.
Victorian Ambulance Union secretary Danny Hill in March told AAP the death of the man during the outage was distressing for the family members involved and paramedics who tried to revive the patient in inner Melbourne.
He said the family struggled to get through to triple zero and later received a call back after their loved one had died.
