⚠️ Telstra's 3G network has now shutdown. Text '3G' to 3498 to check your device. Incompatible handsets will be blocked from accessing the network, more info here
Eventually yes, you will have no choice but to make the switch to nbn™ as traditional fixed-line Internet services, such as adsl and cable, will be phased out.
Once nbn™ rolls out in your area, you have up to 18 months to switch until your existing telephone line and internet connection will be cut off.
