The telcos will cut off up to 60,000 phones that are incompatible with 4G for emergency calls, as Australians are being urged to upgrade their technology in the final days of the 3G network.
Telstra and Optus will begin shutting off 3G from Monday, having first flagged the closure five years ago.
The companies say switching off 3G will allow them to expand their 4G and 5G networks, with the aim of improving the quality of mobile service.
A type of phone that cannot connect to triple-zero on 4G has long been a public safety concern for the federal government, forcing the shutdown to be delayed twice.
There are about 60,000 of these handsets still in circulation that are not enabled with a technology called Voice over LTE.
This means the phones - usually bought overseas or second-hand - use 4G for regular calls and texts, but bump emergency calls to 3G.
There have been fears users would not realise there was an issue until they needed to call the emergency line post-3G.
As a measure to ensure all Australians can make emergency calls, a federal law also comes into effect on Monday forcing telcos to disconnect those handsets for all voice and data services.
The legislation's intention is to be the ultimate prompt for users to upgrade their devices.
"When we start closing the 3G network, we will be required to block some mobile phones," a Telstra spokesman said in a statement to AAP.
"These phones aren't compatible with our network because they depend on 3G for triple-zero calls."
Telcos are also required to provide low-cost options and payment plans for eligible users, something both Telstra and Optus have been offering for months.
Users can check the compatibility of their phone by texting "3" to the number 3498.
As a result of incompatible phones being cut off, Australians are being warned some redundant models are still being sold online.
Seven people in Western Australia have complained to the state's consumer watchdog about spending a total of more than $4000 on obsolete refurbished phones.
"While refurbished phones may be a budget-friendly option, we strongly urge consumers to be cautious before buying one and ensure the retailer they're purchasing from is supplying Australian models," WA's consumer protection commissioner Trish Blake said.
The 3G shutdown will begin in Tasmania on Monday, with the rest of Australia to follow by November 4, a Telstra public statement said.
The federal health department has urged users of devices such as medical alarms and health monitors to check whether they are 4G compatible.
Farmers and rural Australians have also called on the telcos and the government to ensure their mobile service isn't worse post-3G.
Communications Minister Michelle Rowland said Telstra and Optus had committed to coverage equivalence, which would be monitored by the federal government.
