Hi Jess,
There is actually a clever coding system in place which will allow you to do just that. The way it works is, enter a code on your handset and press the call button. Your handset should then tell you if the code has been successful. If so, you will then receive 30 seconds of ringing before it goes to voice mail.
For more details see the following Mobile Fun blog post:
Voicemail Delay Settings - Mobile Fun