error
The phone number you entered has already been registered by someone else.

This is the standard error you’ll see in the Exchange admin center when trying to enable Unified Messaging on an extension that already has it enabled.

When a user departs you’d expect that when you change the user mailbox to a shared mailbox and drop the licensing, Unified Messaging should go. However, in Exchange Online the mailbox will still be Unified Messaging (UM) enabled, and hang onto the extension it had.

You probably won’t even notice this until you go to enable UM on another mailbox using that same extension, which leads to the error at the top of this article.

The first challenge is to find the Shared Mailbox that is holding onto the extension. After connecting to Exchange Online in PowerShell, you can run this command:

get-ummailbox | select name, phonenumber | out-gridview

This will show a nice gridview of all your mailboxes and what UM extension they have. You can search/filter this view to find the cuplrit.

If you want to see which of your mailboxes are Shared and have UM enabled, run this command:

Get-Mailbox -RecipientTypeDetails SharedMailbox -ResultSize:Unlimited -filter {umenabled -eq "true"}

Knowing this mailbox, you’d expect it should be easy to turn off UM. This wouldn’t be too much of a problem if you could just disable UM like you can on a normal mailbox, but in Exchange admin center this isn’t an option at all when it’s a shared mailbox.

Trying to disable UM via PowerShell with the ‘Disable-UMMailbox’ command also won’t work, as you’ll get a license error:

License validation error: the action 'Disable-UMMailbox', 'Identity', can't be performed on the user 'Test User'
 with license 'BPOS_S_Standard'.
     + CategoryInfo          : NotSpecified: (:) [Disable-UMMailbox], RuleValidationException
     + FullyQualifiedErrorId : [Server=SYXPR01MB1901,RequestId=dfc62192-8270-4a65-b582-c7f327d6e7e2,TimeStamp=15/10/201
    9 6:24:33 AM] [FailureCategory=Cmdlet-RuleValidationException] DDB44050,Microsoft.Exchange.Management.Tasks.UM.Dis
   ableUMMailbox
     + PSComputerName        : outlook.office365.com

To fix this, you could use the Exchange admin center GUI along with the Microsoft 365 Portal, but it’s easier to run all the steps required via PowerShell:

First apply a license to the shared mailbox account that includes Exchange Online. You can see what licenses are available to you with this PowerShell command used by the MsolService cmdlet:

 Get-MsolAccountSku

Then, apply a license with this command against the shared mailbox and the AccountSkuID from the previous command:

Set-MsolUserLicense -UserPrincipalName "UPN OF SHARED MAILBOX" -AddLicenses "tenant:licensename"

Once applied, you’ll then need to change the mailbox to a Regular mailbox rather than Shared:

Set-Mailbox "UPN OF SHARED MAILBOX" -Type Regular

After a while, Unified Messaging may drop off by itself if you allocated a license that doesn’t support it (such as Exchange Online Plan 1 or Exchange Online Kiosk, or you can force it off with this command:

Disable-UMMailbox -Identity  "UPN OF SHARED MAILBOX"

Finally you can now enable UM on that other mailbox that was getting the error on the extension being in use. Easily done via the Exchange admin center GUI.

Two last steps are then to reverse what you did – take the license away from the shared mailbox, and make it a shared mailbox again:

Set-MsolUserLicense -UserPrincipalName "UPN OF SHARED MAILBOX" -RemoveLicenses "tenant:licensename"

Set-Mailbox "UPN OF SHARED MAILBOX" -Type Shared