Office 365: Give one user access to all calendar’s

Firstly, connect to Exchange Online Powershell, like described here:

https://docs.microsoft.com/en-us/powershell/exchange/exchange-online/connect-to-exchange-online-powershell/connect-to-exchange-online-powershell?view=exchange-ps

Then copy-paste this to a powershell.ps1 file, like calrights.ps1:

Foreach ($mbx in get-mailbox){

$calendar=$mbx.alias+”:\Calendar”

Add-mailboxfolderpermission -identity $calendar -user call -AccessRights Reviewer

}

• USERNAME is the user you will give the rights spcified, here is is the user “call”
• PERMISSION is the permission level (role) you want to give that user. The options are:
  • Author   CreateItems, DeleteOwnedItems, EditOwnedItems, FolderVisible, ReadItems
  • Contributor   CreateItems, FolderVisible
  • Editor   CreateItems, DeleteAllItems, DeleteOwnedItems, EditAllItems, EditOwnedItems, FolderVisible, ReadItems
  • None   FolderVisible
  • NonEditingAuthor   CreateItems, FolderVisible, ReadItems
  • Owner   CreateItems, CreateSubfolders, DeleteAllItems, DeleteOwnedItems, EditAllItems, EditOwnedItems, FolderContact,FolderOwner, FolderVisible, ReadItems
  • PublishingEditor   CreateItems, CreateSubfolders, DeleteAllItems, DeleteOwnedItems, EditAllItems, EditOwnedItems, FolderVisible,ReadItems
  • PublishingAuthor   CreateItems, CreateSubfolders, DeleteOwnedItems, EditOwnedItems, FolderVisible, ReadItems
  • Reviewer   FolderVisible, ReadItems

Normally Reviewer is enough for ex. 3.party to take telephone calls and check if user is occupied.

NOTE: If permissions for this user have already been defined, this cmdlet will not replace it!

Source: https://www.itjon.com/give-one-user-access-to-everyone-elses-calendar-in-office-365/