Law enforcement agencies track cellphones using devices that intercept signals, including Stingrays or signal-sniffers such as Wolfhounds. Cellphones are also trackable if they are connected to Wi-Fi.
Stingrays, the most common type of device used for surveillance, are an aggressive type of International Mobile Subscriber Identity catcher that police vehicles can carry. They work by posing as cell towers with strong signals that prompt cellphones to connect and relay their locations and identifying information, such as the IMSI number or a serial number. Stingrays impersonate 2G towers, but they can trick phones that use 3G or 4G into downgrading to 2G. Laws prohibit the use of Stingrays without a warrant.
The more passive Wolfhounds and similar devices do not interact directly with cellphones like Stingrays do. Instead, they observe for radio waves that cellphones give off when they attempt to communicate with cell towers. These signal-sniffing devices are wearable and very small.
Wi-Fi is a less effective method of tracking cellphones, because it only works at short range and depends on the cellphone user to make the initial connection. After the connection, the cellphone relays a signal that carries the media access control address, which is exclusive to that phone.