Australians are being urged to upgrade their mobile phones and check their medical devices as public safety concerns delay the 3G network shutdown.
Telstra and Optus jointly announced the 3G closure would be pushed back to October 28.
Both telcos had been preparing to switch off from the end of August but a recent Senate inquiry confirmed thousands of phones would not be able to make triple-zero calls after the shutdown.
The inquiry was also told critical medical equipment, personal cardiac alarms, EFTPOS machines and waste and water infrastructure could be cut off.
A joint statement from Telstra and Optus on Wednesday said the companies would undertake a mass public safety campaign to ensure users were aware they needed to upgrade.
The companies said it was critical customers check handsets are compatible with 4G and 5G and be aware if other devices relied on 3G.
The telcos' executives defended their actions in contacting customers in the years leading up to the shutdown, which was first flagged in 2019.
"We have been communicating to customers about the need to prepare and make the move for almost five years," Telstra chief executive Vicki Brady said.
Optus interim chief executive Michael Venter said the telco "pulled out all the stops" in contacting customers but a final push would drive the message home.
A category of phones that will not connect to triple-zero on 4G has been a major concern.
These devices - often bought overseas or second-hand - use 4G data for regular calls and texts but bump triple-zero calls to 3G because they are not enabled with a technology called Voice over LTE.
Users might not realise their phone is configured this way by the manufacturer until the 3G network is switched off.
There were still 77,000 affected handsets in early August and Optus data revealed many users were international students or migrants.
All telcos have a service for customers to check the status of their device by texting "3" to the number 3498.
Communications Minister Michelle Rowland had foreshadowed her regulatory powers to force a delay, though the decision was ultimately made by the telcos.
"This is a sensible move by both major telcos and the government looks forward to ensuring all Australians have the opportunity to be informed about what the 3G switchover will mean for them," she said.
Opposition communications spokesman David Coleman said the closure had been mishandled.
"The government was too slow to act on the warning signs and significant responsibility for the delay sits clearly at the doorstop of the communications minister," he said.
The inquiry heard Telstra first announced the shutdown in October 2019 and the scale of the triple-zero issue was not clear until February 2024.
Customers are encouraged to find out more on the Telstra and Optus 3G closure pages.
