From Seatlle18 on this thread http://blackberryforums.com/showthread.php?t=28421
But this is a 8700 and I am assuming on Cingular...so maybe not.
Originally Posted by Seattle18
On your phone dial:
Where XXXXXXXXXX is your market's VM number and tt is the time in seconds you want the phone to ring for. I believe valid range is 1-30!
Remember to use the + right before your voicemail number. Do not input more than 30 seconds because you could have some problems of calls never reaching voice mail. Just type it in and hit send/call and you should get some sort of confirmation message. Then try it. I believe the default setting is 15 seconds.