Optus was warned at least five times of a triple-zero call outage, as the telco giant's boss shuts down questions about whether he will resign over the scandal.

A botched firewall update blocked hundreds of triple-zero calls from Optus customers in South Australia, Western Australia and the Northern Territory.

Early investigations appeared to show established processes were "not followed", Optus chief executive Stephen Rue said.

"We are speaking with the individuals involved as to the full technical detail of the network failure," he told reporters in Sydney on Sunday.

"We will leave that for the investigation."

Two customers contacted Optus call centres early on Thursday morning before the outage was fixed after more than 13 hours.

A review has uncovered another three calls over the

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