The Optus outage resulting in the failure of emergency calls has been linked to three deaths. Photo: Bianca De Marchi/AAP PHOTOS

HOW DID THE OUTAGE UNFOLD?

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* A firewall upgrade is initiated by Optus at 12.30am AEST on Thursday

* The upgrade blocks calls for Optus users in South Australia, Western Australia and the Northern Territory

* A customer calls the Optus contact centre directly about 9am to complain triple zero calls are not working

* Telecommunications Industry Ombudsman also refers complaints about the failure to Optus

* These calls were not handled "as would be expecte

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