Directional two-tone elevator sound

Some immutable law, whose name is unknown to me, states that every time you are on a long ride in a lift (as I shall henceforth refer to it), any intermediate stop to let people off is accompanied by some fool trying to get on it for a ride in the opposite direction. I don't particularly blame them - I'm sure they're too busy thinking about Very Important Things, like the economy, or lunch, to look at the lights on the wall.

However! If the lift bell in each lobby was composed of two tones (a low "bong and higher "bing"), they could be played "bong-bing" to indicate a lift going up, or "bing-bong" for a downward journey. Pretty soon, they would instinctively ignore the lift irrelevant to their trip, and everyone's happy.

(I've never heard such bells in a lift lobby, and cursory searching does not reveal such a system, but I won't be surprised if it's already well baked.)