Exchange 2013 Enabling UM Automatically Assigns Wrong Extension

While enabling UM in Exchange 2013 EAC, you might come across an error "Extension xxx is already assigned to another user on dial plan "blah blah" or on an equivalent dial plan".

In the following pic, you can see that extension "149" appears to be already assigned (which it isn't) and you can check that by running the command:

Get-UMMailbox -ResultSize unlimited | where { $_.Extensions -eq "149" }

 

 

The problem is, Exchange UM pulls the extension from the AD Telephone Number properties of the user that you are enabling for UM. As you can see in the following screenshot, "0149" is the last four digits of the user's phone number.

 

 

So, Exchange thinks the extension "149" is already in use. The fix, is to not populate the Phone Number attribute until after you enable the user for UM. If you use a provisioning script (as we do) you'll need to edit it to populate phone number later on.