A simple Lync Management Shell Script does the job:
Get-CsUser | where {$_.LineURI -eq "tel:+649xxxxxxx" -or $_.PrivateLine -eq "tel:+649xxxxxxx"} | format-table -property DisplayName,LineURI,PrivateLine
Granted you can search and filter from Lync Control Panel for Tel Uri, there is no way of finding Private Line URI's from the Control Panel
