That is not a customizable setting in OCS natively.
Take a look at this past discussion for some alternatives:
http://social.microsoft.com/forums/en-US/ucconferencingscheduling/thread/851b7462-0c35-4520-9526-6911953eb2e8/
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS