The SCHEDULE+ FREE BUSY folder is available to any authenticated user. However, Exchange Server 2007 introduced the Availability Service. The Exchange Availability Service allows for access to be configured per user and to control how much free/busy information is shared. This is because Microsoft Exchange Server 2003 and earlier versions publish user free/busy information in the SCHEDULE+ FREE BUSY dedicated public folder.
When the shared mailbox is moved from Microsoft Exchange Server 2003 to Microsoft Exchange Server 2007 or newer, Outlook 2010 or later versions may no longer be able to view availability information by using Open Other User's Folder. Or, you can view that user's free/busy information by opening a new Meeting Request and adding that user in the Scheduling Assistant. To open a shared Calendar in Outlook 2007, the user must grant you Reviewer or higher permission. That feature is not available in Outlook 2007. Outlook 2010 introduced the ability to display another user's free/busy data in a Calendar view when that user's Calendar folder permission is set to Free/Busy. See the folder contact or your system administrator. You do not have sufficient permission to perform this operation on this object.