Optus has admitted that nearly 2,700 Triple Zero calls failed during the company’s disastrous outage in November that left millions of people without coverage and halted connectivity to businesses, train networks, and hospitals.

The outage, and Optus’s lacklustre public response, led to a parliamentary hearing during which then-CEO Kelly Bayer Rosmarin – she resigned shortly after – said only 228 calls to Triple Zero failed while the network was down.

“We have done welfare checks on all of those 228 calls and, thankfully, everybody is okay,” Bayer Rosmarin said at the time.

But late on Tuesday afternoon, Optus admitted the problem with Triple Zero calls was over 10 times as big as Bayer Rosmarin had claimed.

An internal review showed “an additional 2,468 customers” tried unsuccessfully to call Triple Zero during the outage, Optus said in a statement.

Interim CEO Michael Venter said November’s outage was a serious failure from Optus that “did not meet the standards [of its] customers and the community".

“I offer my deepest apologies to all those customers who were unable to access Triple Zero services during the outage and did not receive a follow-up check from us,” he said.

“We are writing to each customer individually to apologise for this and provide the opportunity to discuss their specific circumstances and whether there is anything we can do to assist them further.”

All telcos are obligated to conduct welfare checks on anyone who failed to make an emergency call during a network outage.

Those checks can be as little as an SMS message or phone call that, if unanswered, lead to police being called out to the customer’s property.

Welfare checks were mandated following a 2018 Telstra outage that led to over 1,400 people being unable to reach Triple Zero.

According to a formal investigation into that outage, there was an instance when a person couldn’t call Triple Zero while their house was on fire and instead drove to their nearest fire station to sound the alarm.

Communications Minister Michelle Rowland said Optus’s initial failure to identify that nearly 2,500 extra customers had failed to get through to Triple Zero was “a deeply concerning development”.

“Optus has advised it will commence a process to contact impacted customers and the government has conveyed its expectation this occur expeditiously.”

Optus has not explained how there could be such a massive discrepancy between what it first reported and the updated figures it coughed up two-and-a-half months later.

A spokesperson said the telco will let a third party “undertake a review of our processes supporting our welfare check obligations”.

“We will commit to implement any recommendations of this independent review and share the findings with the [Australian Communications and Media Authority] ACMA and the Senate Committee.”