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:
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:
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.