It's a close cousin of the IMSI catcher, the fake-tower device used for surveillance. Both work by tricking phones into connecting to the strongest signal and sharing information before anything is authenticated.
One of the main things a fake tower can collect this way is your IMSI, the identifier tied to your SIM, but it's far from the only thing your phone gives away. Your phone has several radios that connect to different kinds of signals and share different information.
The Wi-Fi and Bluetooth radios share hardware addresses, which modern phones randomize or rotate to reduce tracking. The cellular radio reveals two identifiers that typically never change: the IMSI, which is tied to your SIM card, and the IMEI, which is tied to your device. The IMSI (International Mobile Subscriber Identity) is a unique number assigned by your mobile carrier when you sign up for your service.
