I've been using the function on the iPhone where if you quickly double press the sleep\wake button when the phone is ringing, it'll decline the call and send it to voicemail. The problem is it sends it to the Telstra service where they record the callers number and send you a SMS soon after containing the number.

If I let the call ring out the caller goes through to my normal recorded voicemail. How do I program the sleep\wake button function to do this?

It appears my voicemail is working fine it's just an issue with the sleep\wake button function. Any ideas how to fix this?