A fourth person has died after being unable to call triple-zero during an Optus outage. Photo: Bianca De Marchi/AAP PHOTOS

Pressure is rising on Optus to be penalised as investigations are promised into an outage linked to at least four deaths.

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A botched firewall update at 12.30am on Thursday blocked hundreds of calls to triple zero in South Australia, Western Australia and the Northern Territory.

An eight-week-old boy and a 68-year-old woman died in Adelaide during the fault, along with a 74-year-old man and 49-year-old in Perth.

Optus was unaware of the outage for triple zero ca

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