In a best-case-scenario, a cell phone’s signal may be picked up by three or more cell towers, enabling the “triangulation” to work. From a geometric/mathematical standpoint, if you have the distance to an item from each of three distinct points, you can compute the approximate location of that item in relation to the three reference points. This geometric calculation applies in the case of cell phones, since we know the locations of the cell towers which receive the phone’s signal, and we can estimate the distance of the phone from each of those antennae towers, based upon the lag time between when the tower sends a ping to the phone and receives the answering ping back.