Just click on the Voicemail Tab on the left hand side. Add a new voicemail mailbox and take note of its number.
Now click on the extensions tab and select the mailbox. This will give your phone a Message Waiting Indicator or stutter dialtone when you have new messages. Now you must turn on Divert for Busy/30s (30s = 30 seconds, you can pick a different value if you wish). Then choose "Mailbox: x" for Divert to.
Now if your phone is busy, or it is unanswered for 30s, the caller will be sent to your voicemail box!