When your cell phone is switched on it send out a signal every couple of minutes to find the cell phone mast that will give you the best signal. The police, phone company, or 911 operator can see which mast you are connecting to, as well as the next two closest masts. This gives them your location to within a very small distance. Sometime a matter of meters.

Many phones also have a GPS chip in them. This lets them track your cell phone with even more accuracy.

Put simply, if you turn your cell phone on the police or the phone company can trace exactly where you are and can direct the nearest squad car to your location. This lets them find people who are trapped in the trunk of a kidnapper’s car or who are hurt but who don’t know where they are or can’t answer to say where they are, as well as prank callers and so on.

If you have a disposable cell then they can track where you are in this manner. If you have a contracted cell then they also know exactly who you are, where you live, and everything else that they need to know about you. Only a fool does something illegal on their own cell these days.