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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Although outdated and not as useful for internet, 2G phones have been favoured for their reliability, low cost and relatively long battery life.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2G, or second generation wireless technology, became commercially available in 1991 and has since been superseded by 3G and 4G technologies.
2degrees stopped selling 2G devices in 2015 and chief executive Stewart Sheriff said most 2G customers had already upgraded their devices.
"We are making this change to better serve the majority of our customers' needs and keep up with their demand for 4G services," Sheriff said.
"Our 4G network offers data speeds that are around a thousand times faster than the 2G network, which was used mainly for calls and texts."
The company said as a general rule 2G only phones were not smartphones.
Customers that could only see a 2G, Edge, E or GPRS symbol in the top right of their screen likely had a 2G device.
The company said consumers could also check their website to see if they would be affected.
Vodafone spokesperson Elissa Downey said the telco still had about a million devices registered on its 2G network and had no plans to turn this off until about 2025.
She said 2degrees customers operating devices on the 2G network could switch to Vodafone to continue using them.
Shutting down of the 2G network has been occurring globally with Vodafone in Australia planning to switch off its network by the end of this month.
Spark does not have a 2G network. It adopted 3G in 2009 when it launched its XT mobile network.