We're somewhat fortunate here in the U.K. in that we now have a universal code which is the same in all areas, at least for all BT-provided lines. The same number also provides ringback and other test facilities.
It's good enough for positively identifying most single lines, but a lot of people don't realize that because it is looking at the CLI number it might not return the correct number on a multiline hunt group where the lines don't have individual directory numbers assigned, or where a business has otherwise had the system configured to deliver their main number on all calls, regardless of the actual originating line.
