umm you talking cell phone triangulation? or reversing phone numbers?
Triangulation you'd have to have control of two cell phone towers... But basically the person would be in the center of two towers in most instances, it would be pulling a signal from two or more towers, and it would be where they cross.
For reversals you can find MANY sites that can do this for you, or if you're like me you have a toll free number you can call up and type in a phone number and it'll say the address associated.
-Giz