SYDNEY (Reuters) -Optus, Australia's No. 2 telecom carrier, said on Sunday that a departure from regular processes on a network upgrade sparked a technical failure that disrupted emergency call services for 13 hours and has been linked to four deaths.

There has been growing outcry in Australia over the glitch, which Optus said occurred during a network firewall upgrade from 12:30 a.m. on Thursday (1430 GMT on Wednesday) until about 1:30 p.m. and potentially impacted 600 customers.

The government on Friday said it would investigate the company's "unacceptable" failure, and Optus said the next day it would cooperate with any effort to look into the incident.

Optus CEO Stephen Rue said in a statement on Sunday the company's own initial investigation has shown that established processes were not followed on the upgrade.

"As to the full technical detail of the network failure, we will need to leave that for the investigation," Rue said.

Five customers contacted Optus' call centre to report the outage but their concerns were not escalated, he said.

"That is clearly not good enough ... I want to reiterate how sorry I am about the very sad loss of the lives of four people, who could not reach emergency services in their time of need," he said.

Two of the dead were identified as an 8-week-old boy and a 68-year-old woman, South Australia police said. The other two fatalities were men aged 74 and 49, police in Western Australia said.

Rue said on Friday Optus had fixed the fault and would make the results of its investigation into the incident public.

The incident comes less than a year after Optus, owned by Singapore Telecommunications, was fined A$12 million ($7.9 million) by regulators for failing to provide emergency call services to thousands during a nationwide outage in 2023.

Optus also suffered a cyberattack in 2022 that affected the data of around 9.5 million Australians.

Former CEO Kelly Bayer Rosmarin resigned in the wake of the earlier incidents, and Rue took over in November 2024.