How to fix the calendar free/busy or calendar sharing issue?

    Calendar is one of the top most part in the production environment, by using that we can schedule call with the person whom we would like to have meeting or call. This calendar feature is can be shared to inside and outside of the organization users. Mostly all the organization make all user's calendar is visible for everyone in the organization.

    But in some cases we may wanted to share our organization users calendar to our customers/partnership organization. In such cases exchange administrator has to make trust the partner domain and define sharing policy for their domain in office admin center. This something can be done by using GA Role.

    In that policy we can limit which kind of calendar information to shared with the partnership organization. If this pre-requisite met, then the federated partner can view our user calendar. The same kind of policy has to be made from partner organization as well.

Unable to see Single user's calendar inside and outside organization ?

       Before start troubleshooting make sure you have proper calendar sharing policy and Auto discover is working fine. 

• Use Microsoft connectivity test tool to check auto discover functionality.
• First check the impacted user calendar can be viewed by any other users.
• Check the same in the outlook web app as well.
• When you try to check the availability of the impacted user you may see shaded bar in his whole calendar.

        Some VIP user don't wanted to show their calendar information to all organization users. So we needed to verify the person who wanted to access the impacted user's calendar has enough permission.

This can be done from impacted user's outlook or from exchange PowerShell.

Calendar permission check from user's outlook:

• open--> Outlook application --> look for the "calendar" option

• Open "Calendar"--> Right Click on the Calendar.

• Right click--> properties  --> Then go to "permission".

• There you can see the permission granted of the impacted user's calendar. 
• If default or Anonymous available in the permission output, all good from permission end. 
• Else you may have to provide permission by using Permission tab "Add"options.

Calendar permission check from PowerShell:

• Open --> Exchange online powershell.
• Connect-EXOPSSession --> Then enter your exchange admin credential.
• Run the below cmdlet to check the calendar permission of a user.
• Get-MailboxFolderPermission -Identity "MailAddress" | fl
• If default or Anonymous available in the permission output, all good from permission end. 
• Else you may have to provide permission by using below cmdlet,

Add-MailboxFolderPermission -Identity "MailAddress" -User "Mailaddress of the viewer" -AccessRights Reviewer

Calendar Items count check:

• If all permission are okay, the excess of calendar count causing this issue.
• Best practice is to have maximum 1500 Entries in a calendar. When user having recurring meeting and old meeting items it would increase calendar count.
• The count can be check from Users outlook and PowerShell as well,

From outlook:

• open-->outlook application --> look for the "Calendar" option

• Open calendar -->  On the top Rippon you can see option called "view".

• Click the view --> Open change view --> change the view to "List".
• There you can find the "Total calendar items count". (I don't have any calendar entry in this screenshot, so it shows blank).

From PowerShell:

• Open --> Exchange online PowerShell
• Connect-EXOPSSession --> Then enter your exchange admin credential.
• Run the below cmdlet to check the Calendar items count.

Get-MailboxFolderstatistics -Identity "Mailaddress" -FolderScope Calendar| ft -property Name,ItemsInFolder

• If the calendar count is more than 1500, ask user to remove old and unwanted calendar entry.

Post this action issue will be fixed if still issue persists raise a ticket with Microsoft, the issue caused because of database or calendar corruption Microsoft only can fix. The same thing is applicable for on-prem and hybrid Environment as well.