Attempting to enable a user for Exchange UM displays the error message: “Extension xxx is already assigned to another user on dial plan UMDialPlan or on an equivalent dial plan.”

Problem

You attempt to reassign an Exchange UM extension that was previously assigned to a user who you have disabled for UM but receive the following error:

error

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

You try using the Get-UMMailbox cmdlet to list all of the users and review which one currently has the extension assigned:

Get-UMMailbox | Format-Table -Wrap -AutoSize

… but do not see the extension listed in Extensions column:

You review the attributes for the user who was previously assigned the extension but do not see any reference of it in the Exchange attributes:

The Lync / Skype for Business msRTCSIP-Line attribute is confirmed not to exist for the user either:

Solution

One of the possible causes of this issue is if the user who previously had this extension assigned still has the EUM email address with the extension as the value:

To correct this issue, remove the email address from the previous user.