No Stupid Questions

In places where GPS isn't available, in an underground transit station for example, it really seems like a no brainer to just use the cell towers in range to triangulate your location.

The police can already track the location of your phone using cell towers. That's how 911 knows where you are when you call them (AFAIK). Why isn't it just available for the phone itself to figure out its own location so you can use it to navigate?