Restore: Default Calendars on Restored Deleted Mailboxes Blocked

If you are restoring the mailbox for a user who was previously 'hard' deleted (no longer within the recycling folder) that mailbox may have the same name but Microsoft will assign it a new User ID. 

Microsoft creates a new calendar by default for any new user (new User ID). The newly generated default calendar will have the same name as the original mailboxes' default calendar. Microsoft will block the attempted restoration of the calendar and associated events from overriding the new default mailbox.