I believe what you're looking for is Pescado's Phone Hack.
http://www.moreawesomethanyou.com/smf/index.php/topic,1479.0.htmlFEATURES ADDED:
1. Set Ringer - Set the number of rings you want your phone to ring when an
incoming call is received: 5, 10, 15, the default 20, or even OFF, which
means you will not be notified or receive any incoming calls.