Optus says it is working with communications technology company Ericsson to determine the cause of an outage of a tower in Dapto, NSW, on the weekend that led to emergency services calls in the area failing to go through.
It comes as Prime Minister Anthony Albanese said the company had "failed customers" and needs to work to restore customers' trust.
On Sunday, an outage occurred linked to a mobile phone tower in the Wollongong suburb, affecting around 4,500 users in the area between 3am and 12.20pm.
Optus has apologised to users who were affected. A spokesperson said Optus had confirmed with police that those affected were "OK" and the issue had been fixed.
On Tuesday, Optus said Ericsson's equipment "did not appear to operate as it should" and that it was working with the provider to investigate.
Optus said initial assessments of Sunday's outage indicated 4G services in a tower at Dapto were not operational.
The tower appeared active on the network, but calls attaching to it were impacted and did not transfer to other networks, the company said.
"Optus' ability to detect the outage was impacted as the Ericsson equipment in the cell tower did not alarm that 4G services were not operational," it said.
"We are assured by Ericsson that they have their global product development unit urgently analysing this."
Second triple-zero outage in less than a fortnight
Sunday night's outage is the second such incident in less than a fortnight for the telco, after it suffered an outage on 18 September that impacted parts of South Australia, Western Australia, the NT and NSW.
Around 600 calls were disrupted, and the outage has been linked to the deaths of at least three people. That incident is the subject of an Optus probe and federal communications watchdog investigation.
Optus has claimed human error caused the first outage after no one diverted calls to a separate part of the core network, as required.
"Standard processes were not followed," Optus chief executive Stephen Rue said at the time.
Singtel chief executive Yuen Kuan Moon and federal Communications Minister Anika Wells met in Sydney on Tuesday, along with Rue and chair John Arthur.
They discussed technical details about the outages and ways to restore trust in Australia's second-largest telco and the triple-zero system.
Speaking to reporters, Yuen said of the first outage: "It's a people issue and it takes time to change and transform the people."
Arthur similarly told reporters: "The incident on the 18th was a process-related incident. People made mistakes."
'Systemic failures'
On Wednesday, the Communication Workers Union (CWU) said Optus laying the blame for the outages on frontline workers was "disgraceful" and an attempt to absolve the company's "systemic failures".
"How Optus' most senior bosses can stand up and openly blame frontline staff for what is clearly a systemic failure is beyond comprehension," CWU national secretary Shane Murphy said in a statement.
"The company has a responsibility to the Australian people to ensure that there are water-tight systems in place to ensure that when they need to call triple-zero, they can."
Murphy said the telco attributing blame to frontline workers was a "disgraceful attack" on people "attempting to do their jobs under incredibly difficult circumstances", citing offshoring and outsourcing.
"If the system can’t carry a triple-zero call, that's a boardroom failure, not a 'people failure.'"
Speaking to reporters in Sydney on Wednesday, Albanese said it was "in Optus' interests to restore confidence" after it "failed customers".
"They need to be transparent … This should never have occurred," Albanese said.
Wells said on Tuesday that Singtel was accountable for Optus' failures in making sure calls to emergency services went through.
The communications minister said she had asked Optus to "find a way to have an external account in their systems so that Australians can have confidence ... rather than just hearing from Optus again that it will be fine".
Arthur said Optus would appoint an independent, external party to ensure accountability.
— With additional reporting by the Australian Associated Press.
