Attempting to enable a user for Unified Messaging in Exchange Server 2016 fails with: "Extension xxxx is already assigned to another user on the dial plan UMDialPlan or on an equivalent dial plan."

Problem

You attempt to enable a user for Unified Messaging in Exchange Server 2016 but receive the following error:

Extension xxxx is already assigned to another user on the dial plan UMDialPlan or on an equivalent dial plan.

You know that this extension was previously assigned to another user so you search for the previous user and can confirm that Unified Messaging for the user is disabled:

Reviewing the properties of the account’s email address properties confirm there are now EUM addresses:

Executing the following cmdlets does not display this extension assigned to anyone in the dial plan:

Get-UMMailbox | where { $_.Extensions -eq "9533" }

Get-UMMailbox | Format-Table -Wrap -AutoSize > C:\UMExt.txt

You proceed to review the msExchShadowProxyAddresses attribute in the previous user’s AD user account’s properties’ Attribute Editor:

You can see that an eum address with the extension and proceed to remove it.

However, this does not correct the issue as the same error message is thrown.

Solution

I can’t take full credit for the solution but figured it would be worth blogging since it took over an hour for me to locate a cmdlet that was able to find the find the offending user.  This cmdlet can be found on this forum post:

https://social.technet.microsoft.com/Forums/en-US/55dc00d5-2301-49d0-9a02-482d237c339b/exchange-server-2013-enableummailbox-error-extension-2909-is-already-assigned-to-another-user-on?forum=exchangesvrunifiedmessaging

The cmdlet was:

Get-ADobject -filter * -Properties name,msRTCSIP-Line,telephoneNumber,proxyAddresses | Select-Object name,msRTCSIP-Line,telephoneNumber,@{Name="proxyAddresses";Expression={[string]::join(";",( $_ | Select-Object -ExpandProperty proxyaddresses ))}} | Where-Object { $_.proxyAddresses -like "*9533*" } | Format-table -Wrap -AutoSize

This user was also had Unified Messaging disabled:

However, the eum attribute was visible in the properties of the email address settings:

Removing the attributes corrected the issue.