Ever been in my shoes;
About to enable a user for enterprise voice, but you're not allowed as the number is already in use?
Only problem is; You could swear the number isn't in use any more, as you removed a user with that number just a few days ago. With the exact same number.....
Well, here is something I discovered in a deployment at a customer site:
When you remove enterprise voice from a user, you do NOT remove the LineURI property from the user. So when you try to reuse the same number, it is still listed as in use.
You have to remove the assigned number prior to disabling the enterprise voice feature on a user, for it to truly be freed.
What was that? You don't remember which user you edited yesterday (or a few days ago)?
Here is quick tip. Launch powershell, and type the command: "Get-CsUser | ft -Property name, LineURI" This will give you a list off all of your CS users, and their corresponding phone numbers.
Just look for the number you are trying to add to another user, and identify to which user it already belongs to. Then move over to the control panel, locate the user, enable it for voice, remove the number, and disable the voice again...