International students, migrants and regional Australians are being urged to ensure their phones are 4G compatible, as more than 200,000 devices won't be able to make triple-zero calls when the 3G network shuts down.
Telstra is due to switch off 3G on August 31, having extended its original June closure deadline, while Optus will shutdown from September.
The telcos have long flagged the end of the ageing network, but there are still 205,000 mobile phones that are incompatible with 4G, according to the latest industry-wide figures provided to the government.
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 may not realise their phone is configured this way by the manufacturer until the 3G network is switched off and they need to call the emergency line.
Optus research has identified suburbs and regions with the highest number of incompatible devices, where many residents are native Mandarin, Cantonese, Arabic, Vietnamese and Korean speakers.
Some areas may also have large cohorts of international university students.
The city suburbs with the highest number of affected devices on the Optus network are Melbourne's CBD and Sydney's Macquarie Park, Marsfield and Millers Point.
Top regional areas are Port Hedland, Western Australia, Cairns and Sarina in Queensland and Griffith and Orange in NSW.
Optus has been targeting these areas through social and local media.
"Optus is encouraging customers in these suburbs who are receiving messages from us to act on the advice and replace their device," Optus executive Andrew Sheridan said in a statement.
Both Telstra and Optus have a service for customers to check the status of their device by texting "3" to the number 3498.
Telstra has introduced a recorded message on non-emergency outgoing calls advising affected customers they need to upgrade their phones.
It has also given 12,000 phones to people struggling to upgrade, including remote and elderly users.
About five per cent - or 11,000 - of the affected devices are using the Telstra network, regional general manager Chris Taylor said.
"The measures we have in place to inform, prepare and assist our customers to make the transition from 3G are working," Mr Taylor said.
"Telstra is comfortable that our ongoing actions will help minimise any customer devices which remain active."
The telcos' estimates on affected devices have fluctuated from one million in April, to 400,000 in early May, then 530,000 in mid-May.
Though the companies established a working group to better inform customers, the number of incompatible devices is still too high, Communications Minister Michelle Rowland said.
"The Albanese government supports the 3G switchover, but it needs to be done in a safer way," Ms Rowland said in a statement.
"The government will consider potential regulatory intervention if it is necessary and in the public interest, subject to the required consultation and procedural processes."
Both telcos said they have been improving their 4G and 5G coverage in the lead-up to the 3G closure.
AREAS WITH THE HIGHEST NUMBER OF AFFECTED PHONES ON THE OPTUS NETWORK:
Metro:
* Melbourne CBD (north), Victoria
* Macquarie Park/Marsfield, NSW
* Sydney (north)/Millers Point, NSW
* Parramatta, NSW
* Melbourne CBD (west), Victoria
* Hoppers Crossing (north), Victoria
* Box Hill, Victoria
* Adelaide, South Australia
* Blacktown (east)/Kings Park, NSW
* Auburn, NSW
Regional:
* Port Hedland, Western Australia
* Cairns, Queensland
* Griffith, NSW
* Sarina, Queensland
* Orange, NSW
