Telstra announced the 2G shutdown two years ago but some MVNOs continued to sell budget 2G handsets and SIM cards, warns finder.com.au telco editor Alex Kidman. While these resellers now have access to Telstra's 3G and 4G networks, some customers are still reliant on 2G and might not realise that Telstra's shutdown affects them.

"You can't assume that everything is fine just because you're not a Telstra mobile customer," Kidman says. "If you're with some of Australia's other mobile brands — including two of the big supermarket chains in ALDI and Woolworths — you're actually using Telstra's network so the 2G shutdown might affect you."

"If you buy a new phone every few years you've probably got nothing to worry about, but there might be a problem if you've stuck with an old phone — or if you've slipped a SIM card from a budget MVNO into your old 2G Nokia phone and handed it to a relative to use as an emergency phone. Now is the time to check."

Affected customers may see "2G", "GSM", "E", "EDGE" or "GPRS" displayed alongside the signal strength on their handset, although some pre-3G handsets do not display the type of network they're using. In most cases these handsets are only 2G-capable and will stop working with the Telstra network, and its resellers, on December 1.

Alternatively the handset might be 3G/4G-capable but hampered by a 2G-only SIM card if the telco failed to issue a new SIM card when upgrading from a 2G handset. In this situation, the telco will provide customers with a new 3G/4G SIM card free of charge, whilst keeping the same phone number.