On August 31 2012, Telstra will shut down its shared 2100Mhz 3G network.
Telstra currently operates this network in metropolitan areas via a sharing agreement with Vodafone Hutchison Australia (VHA). The existing agreement with VHA is being terminated, and the network will be decommissioned. The spectrum will be made available for future services, such as 4G.
Telstra is advising that all customers still using 2100Mhz 3G devices or handsets - and it is unlikely that there are many of them - will be unable to access certain features once the network is switched off. If for whatever reason you are a Telstra customer using a 2100Mhz device, you'll be tough out of luck, and will need to either purchase a 850Mhz device (Next G), or move to a rival carrier, if you are particularly attached to your old device.
Telstra hastened to add that calls and SMS will remain unaffected.