Telstra has delayed the shutdown of its 3G network over concerns it may stop thousands of customers making emergency calls.
The national telco postponed the network switch to August 31 - two months later than planned - to give customers more time to upgrade their devices.
It's estimated about 200,000 devices across the country still rely on the Telstra 3G network.
That included about 16,000 devices that were 4G enabled but still used the 3G network to make triple zero calls, Telstra said.
"If you're still using an impacted device after August 31, you won't be able to call triple zero for emergency help," Telstra CEO Vicki Brady said on Monday.
"We know some of our customers are still working through the steps they need to take to upgrade their devices.
"That's why we've decided to provide more time and further support."
Most of the 16,000 mobiles unable to call triple zero after the switchover were from overseas and not purchased through the telco.
Telstra said it would do everything it could to help those customers transition to other devices.
"These devices have come from the grey market overseas but we have to support them," Telstra executive Channa Seneviratne told AAP.
It is possible after August 31 the telco industry will block the overseas bought devices from using networks, similar to a move made in the United States.
Telstra said no single demographic had not upgraded their phone, with customers of all ages affected by the shutdown in both cities and regional areas.
Federal Communications Minister Michelle Rowland warned customers their device may appear to be operating normally after August.
She urged customers to ensure they were upgraded, warning people may only find out when they go to make an emergency call.
"Australians may only realise there is an issue with the device when they can least afford it," Ms Rowland said.
Affected devices include mobile phones, landlines, farm machinery and medical devices.
The federal government has set up a working group featuring Telstra, Optus, TPG and the Australian Mobile Telecommunications Association ahead of the 3G shutdown.
"The government will keep working with the working group we helped establish to ensure impacted Australians understand the steps they need to take before the switchover occurs," Ms Rowland said.
Opposition spokesman David Coleman slammed the management of the 3G shutdown, which was first announced by Telstra in 2019.
"This decision underscores Ms Rowland's hapless handling of the key issue that some 4G phones will not work after 3G shutdown," he said.
The delay was welcomed by rural Australians, especially farmers with machinery operating on the 3G network.
"With many farmers often working alone and a long way from help, switching off the 3G network without proper community engagement and support would spell disaster for many in the bush," NSW Farmers spokeswoman Deb Charlton said.
Telstra needed to better educate and support rural Australians before the change, she said.
Customers who have affected devices will hear a message on outgoing calls to remind them to upgrade.
People can also text '3' to 3498 to find out if their handset is impacted.
