Symptom:

You receive the following error when executing the New-CsCommonAreaPhone command via PowerShell.

New-CsCommonAreaPhone : Filter failed to return unique result, "[LineURI : tel:+15555555555] [PrivateLine : tel:+15555555555] "
At line:1 char:1
+ New-CsCommonAreaPhone -LineUri "tel:+15555555555" -RegistrarPool "lync.mydomain ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : NotSpecified: (:) [New-CsCommonAreaPhone], InvalidOperationException
+ FullyQualifiedErrorId : System.InvalidOperationException,Microsoft.Rtc.Management.AD.Cmdlets.NewOcsCommonAreaPhoneCmdlet

Solution:
This means that the phone number has been assigned to either another user or response group. Make sure the number is not in use by Lync.