Hundreds of Australians were unable to make emergency calls during the Optus outage and the telco still doesn't know why.
Optus boss Kelly Bayer Rosmarin has revealed 228 calls to triple-zero failed to go through during a 12-hour outage that affected internet and phone services to millions of customers on November 8.
"We absolutely believe the triple-zero system should have worked and it's critical for all Australians that the system can be relied upon," she told a Senate inquiry on Friday.
"(But) we don't manage the triple-zero system, it's a very complex system that involves all the carriers, it involves the device manufacturers."
Asked by committee chair Sarah Hanson-Young if Optus knew why customers were unable to make the calls, Ms Bayer Rosmarin said they had conducted some inquiries but couldn't fully investigate due to "complex relationships".
"It's too early to tell where the issue actually occurred," she said.
"The triple zero system is supposed to be able to pick up the traffic when we have an outage like this."
Senator Hanson Young accused Optus of attempting to "share the blame" when it should take responsibility, apologise and accept a penalty.
"It's not anybody else's fault Optus customers couldn't call triple-zero, surely it's Optus's fault," she said.
The Optus chief also revealed the outage was so unusual there had been no emergency planning for an event of such scale.
"It's not something we expected to happen," she said.
During the outage, millions of individuals and businesses were unable to make calls, access the internet or complete transactions, and many did not find out the extent of the issue until hours later.
"You provide a service to over 10 million people and not just individuals - government agencies, emergency services, businesses - and all they got for hours, was a couple of lines that said, 'sorry our service is out we're working on it," Senator Hanson-Young said.
"For a communications company, the communication is pretty lousy."
The company later apologised and customers - including businesses that lost thousands in sales - were offered 200GB of extra data, or free data on weekends if they were on prepaid plans as a "gesture of thanks for their ongoing support and patience".
Ms Bayer Rosmarin said Optus had since received inquiries from 8500 customers and small businesses with roughly $430,000 of compensation under discussion.
The telco has already provided $36,000 across all claims but it is unclear if this was offered in cash or services.
The telco initially blamed the outage on a routine software upgrade but its updated submission to the inquiry reveals it was the result of its routers' default settings.
The Cisco routers automatically self-isolated to protect themselves from an unexpected overload of IP routing information.
It took longer than expected to restore because some routers needed to be physically rebooted, requiring staff to deploy to a number of sites across the country.
Rumours of Ms Bayer Rosmarin's resignation have swirled around Optus since the outage and though she has dismissed claims politicians and members of the public continued to call for change.
"Given you didn't know the weakness in the network, given you haven't responded to the customers well, given you haven't reflected your staffs attitude - isn't it time for new leadership?" Nationals Senator Ross Cadell asked.
Ms Bayer Rosmarin said she would take the feedback on board as the company attempts to move forward.
The outage came just over a year after an Optus data breach that compromised the information of millions of Australians and caused the Medicare, licence and passport numbers of 10,000 customers to be stolen and leaked online.
