Australian telco Telstra has blamed its Triple Zero outage earlier this month on software that unexpectedly failed while medical support devices were logging into its network.

Triple Zero (000) is Australia's main emergency service phone number.

Addressing the 90-minute outage, which took place on March 1, Telstra said that it was down to a combination of a "technical fault, an issue in our backup process, and a communication error that occurred in the heat of the moment."

During the incident, 148 calls failed to transfer, with one man from Victorian dying of a cardiac arrest in this period.

The incident, which happened at around 3:30 am, was picked up by Telstra's Triple Zero team, who identified an issue with Calling Line Identification (CLI) not appearing for calls coming into the service.  

CLI is critical as it provides the team with the location and phone number of the person calling and is needed to transfer that call to the relevant emergency services operators.  

During the outage, the company said it had to enact its backup process, which involves asking for the caller’s location and then manually connecting them to the relevant emergency service.  

Telstra said it was successful for 346 of the 494 calls made during the incident, while 127 of the remaining 148 calls had to go through a manual email transfer and callback process. This was because some of the phone numbers for the relevant emergency services stored in the backup database were incorrect. 

Following discussions with Telstra's Triple Zero team, the remaining 21 callers advised they did not require emergency assistance, added the telco.

"Let me reinforce that the series of failures that occurred on March 1 are unacceptable," said Vicki Brady, CEO, Telstra. "The 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.  

"As CEO of Telstra, I apologize to everyone who tried to call Triple Zero during this issue, and in particular, the family of a man who suffered a cardiac arrest and tragically passed away."

Telstra added that it has conducted an investigation to identify the cause of the CLI issue, and reproduced the issue in its lab environment.

The telco said that it will finish testing and the deployment of the software change required to fix the fault in the CLI platform by early April.

Just last week, the Australian government said it's creating a working group, made up of Telstra, Optus, TPG, and the Australian Mobile Telecommunications Association (AMTA), to manage the 3G to 4G switchover "in a safer way," amid concerns around emergency calls.

In a statement, the government said on March 17, that the Working Group will focus on a subset of 4G handsets configured by the manufacturer to use 3G for calling 000, despite otherwise working over 4G to make voice calls.

An estimated 740,000 Australians with 4G handsets could be impacted by the shutdown of 3G.