thanks for your reply Diane.
Yes, Our users sync their exchange emails with mobile devices.
One of the users experiencing this problem is syncing with his mobile device but another user having this same problem does not.
I have come across this problem recently where calendar appointments are created and the default free/busy time is set to "busy". After a month or so, checking the same appointment, the free/busy time somehow changes to "free".
It is becoming more frequent with users I support and...