Thousands of 2degrees customers will have to upgrade their phones this week or change to another provider as the telco switches off its 2G network.
Devices that could connect to 3G or 4G would not be affected but those that could only connect to 2G would cease to operate from Thursday the company said.
Although outdated and not as useful for internet, 2G phones have been favoured for their reliability, low cost and relatively long battery life.
According to a spokesperson, the number of 2G-only devices left in use had been declining with a number of those remaining likely to be EFTPOS terminals or other similar products.
2G, or second generation wireless technology, became commercially available in 1991 and has since been superseded by 3G and 4G technologies.