Try this...
To extend your voice mail all you need to do is:
Obtain your voice mail number by dialling *#67# from your phone. The number will then be displayed on their screen.
Dial 61* your Voice mail number**number of seconds# followed by the Call or Send Key on the phone.
Number of seconds - this is the number of seconds to which you wish to extend or reduce the delay before going to voice mail. It can be between 5 and 30 seconds. i.e. 5, 10, 15, etc.
Example: **61*+440717941123456**25#.
The change should go through straight away.