if you have the address of the person, there are #'s you can call that will tell you...all you gots to do is use some social engineering....be friendly....ask them how their day was, tell them that yours was alright, but you've got a problem, see you're out on the job, and you're across the street from (house address here), and you are workin on the phone lines, and you need the phone # of the house to figure out which line is which.....
i think there's an article about it somewhere...i will try to find it...
edit: a quote from the actual
anarchist cookbook (online), so may be incredibly inacurate.
25. Unlisted Phone Numbers by The Jolly Roger
There are a couple of different ways of doing this. Let's see if this one will help: Every city has one or more offices dedicated to assigning numbers to the telephone wire pairs. These offices are called DPAC offices and are available to service reps who are installing or repairing phones. To get the DPAC number, a service rep would call the customer service number for billing information in the town that the number is located in that he is trying to get the unlisted number of. (Got that?) The conversation would go something like this: "Hi, Amarillo, this is Joe from Anytown business office, I need the DPAC number for the south side of town." This info is usually passed out with no problems, so... if the first person you call doesn't have it, try another. REMEMBER, no one has ANY IDEA who the hell you are when you are talking on the phone, so you can be anyone you damn well please! When you call the DPAC number, just tell them that you need a listing for either the address that you have, or the name. DPAC DOES NOT SHOW WHETHER THE NUMBER IS LISTED OR UNLISTED!! Also, if you're going to make a habit of chasing numbers down, you might want to check into getting a criss-cross directory, which lists phone numbers by their addresses. It costs a couple hundred bucks, but it is well worth it if you have to chase more than one or two numbers down!