I know this works for both Telstra and 3G. My phone is with Telstra and I was getting very frustrated because whenever I got a call it would ring like 4 times then go to message bank. Most of the time this would happen before I even had a chance to get to the phone.
To make the phone ring for twice as long simply type in:
Then hit the call button. Some weird stuff will come up on the screen, just OK it. Then try calling the phone, it will ring much longer!