There is a case when a user wants to block his/her calendar (free+busy) information not to be announced to users in your organization.
They can block it by selecting calendar's permission to "None" for the default setting.
It gets set back to "free+busy" after about 24 to 48 hours later. it is done by BES (4.1.5 or later).
Disable the free+busy look up capability to prevent the problem.
Here is an instruction:
* if you do it, users with handheld software version 4.5 or later can not perform free+busy lookup