When I was on the Telephone Repair Cust.Svc. desk, we would have callers press their touch-tone pad keys and if we got 'click, click' we knew that it was reversed. Since it's been an eon ago I don't remember exactly how the phone calls were established, either us calling them on the reported line or them calling us but I think they'd report the phone line from another phone and we'd attempt a call to the errant phone which would ring fine and be usable, as Bjarney describes above.
