At a client site yesterday, the help desk were provisioning new users with phone extension and for one they came across the error below.
The user was already assigned this extension in Lync and AD had the correct information. Normally when an extension is in use they would not have been able to add the extension in link.
In order to resolve this issue – I ran the Get-UMMailbox command. I piped the output to a text file and found that the extension above was still in use on a disabled user. Once removed we were able to assign the extension to the new user.