Half a million Australians may have phones that won't connect to triple zero after the 3G network shuts down, a situation the federal government says is "deeply concerning".
The nation's 3G network is in its final months of operation, with Telstra switching off on August 31, having recently extended its deadline.
Optus will shut down from September, while TPG Telecom/Vodafone began its closure in January.
Though the end of 3G has been flagged for years, there are up to 530,000 Australians with devices incompatible with the 4G network, according to the latest industry figures.
These devices - often bought overseas or on the grey market - may 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 until 3G is switched off.
The telcos' estimates on these devices have changed from one million in April, to 400,000 earlier this month, and up to 530,000 this week.
Communications Minister Michelle Rowland said the fluctuating figures were "deeply concerning".
The telcos have been contacting affected customers and those users should take action, she said.
Ms Rowland said Telstra's decision to delay its shutdown was welcome, while an industry working group is regularly updating the government.
"Options exist under law for the government to consider regulatory intervention - including proposals for delays to planned switchovers, subject to required consultation and procedural processes," she said in a statement.
Both Telstra and Optus have launched a service for customers to check the status of their device by texting "3" to the number 3498.
Optus executive Harvey Wright said the company has ramped up its efforts to keep users connected, launching the text service this week.
Customers can also be assured 3G will not be switched off where it is the only available network until 4G is made available, Mr Wright said.
"The number of sites like that are very small and we're confident that we will be able to maintain the coverage that we've got in both metro and regional throughout this process," he told AAP.
Announcing its delayed shutdown earlier this month, Telstra said customers yet to upgrade will soon hear a short recorded message on outgoing calls reminding them to upgrade their device.
"While there is now a little more time, please don't delay," Telstra's announcement said.
"Our 3G network is closing soon, and it is important you act now."
The 3G shutdown will also affect medical devices, farm machinery and EFTPOS terminals.
Graeme Hughes, a consumer commentator and director of Griffith University's Co-Design Lab, said those devices were of particular concern.
"I would like to see an extension of the shutdown," Mr Hughes told AAP.
"There needs to be a collective approach, with co-messaging from government and Optus and Telstra ... which enables the consumer to really understand what they need to do to not be at risk."
