Failed triple zero calls during Optus outage 10 times higher than reported

Former Optus CEO Kelly Bayer Rosmarin told a Senate inquiry that same month that only 228 calls from mobiles to emergency services couldn't connect.

But the telecommunications provider said on Tuesday that it had since discovered an extra 2468 customers who tried to make triple zero calls from its network during the November 8 blackout did not reach the emergency service centre and did not have welfare checks undertaken on them.

Optus interim CEO Michael Venter apologised to customers who were unable to access triple zero services on the day.

"There is nothing more important to us than the safety and security of our customers, but regrettably on 8 November we did not meet the standards our customers and the community expects from us," Mr Venter said in a statement.

"... 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.

"We know we let our customers down and our entire team is committed to addressing all learnings from the outage.''

In a statement, Minister for Communications Michelle Rowland said the revelation at least 2697 calls failed to reach emergency services was "a deeply disturbing development given the critical importance of the triple zero service".

"Optus has advised it will commence a process to contact impacted customers and the government has conveyed its expectation this occur expeditiously," she said.

The Australian Communications and Media Authority is investigating whether Optus met its obligations to provide emergency call services under the Telecommunications (Emergency Call Service) Determination 2019.

The minister said the authority would consider the updated failed call figures as part of its independent investigation.

In response to the revelations, Optus said there would be a separate review of how it handled its welfare check obligations. It would share the results of this review with the authority and the Senate inquiry.

The 12-hour outage last year left 10 million individuals and businesses, including some hospitals, without phone connections or internet services. The company blamed a software upgrade for the blackout.

Ms Bayer Rosemarin told the Senate inquiry at the time the company's technical teams were still looking into why some triple zero calls couldn't connect like they were supposed to.

"The triple zero system is meant to be able to pick up the traffic when we have an outage like this," she said.