I actually thought the same thing, but usually it's not a single rule. You have DigitMap and OutboundCallRoute in Phone setup; that usually refers to several other DigitMaps (SP1, SP2, Line; not to mention user-defined DigitMaps). What I would greatly appreciate is "debug mode" for digit maps. You make a call (either through the phone, or on one of the pages) and get all rules resolved.