…called a Stingray. Your phone thinks this device is a cell tower, so it links up with it. Once linked, the police can intercept your phone’s location, serial number, and other private information you would rather them not have access to. Oh, and they get this devices from the FBI, who require that the police first sign a non-disclosure agreement about the Stingray’s existence or use. They can’t even reveal the existence of a Stingray to a judge, without the FBI’s prior permission. I doubt the cops are complaining too much about that aspect.
This Police State tech was sold as a means of tracking terrorism suspects, but since there’s not much terrorism happening at all in the US, the cops use their Stingray to track ordinary criminals, and possibly non-criminals.
The ACLU has an illuminating graph on the known police departments currently using Stingray tech, but it probably dramatically underrepresents their use, given the secrecy surrounding the acquisition and use by departments of this spy tech.