A cell-site simulator (sold under brand names Stingray, Hailstorm, Crossbow, Triggerfish; built by Harris Corporation, KEYW, Digital Receiver Technology, and others) is a suitcase-sized device that impersonates a legitimate cellular tower. Phones in range automatically register with the strongest signal, surrendering their IMSI, IMEI, and approximate location to the operator of the device.
What it means in practice
The device is passive from the user perspective: there is no notification, no warning, no indicator on the phone that it has registered with a fake tower. The operator, typically law enforcement or military, gets a list of every phone in the radius along with identity numbers and signal-strength-based location estimates. Newer generations can also intercept calls and SMS in 2G mode (which is why most carriers and security advocates push to disable 2G fallback in phone settings). The device affects every phone in the radius, not the targeted one alone. Documented uses span counterterrorism, narcotics enforcement, immigration sweeps, protest surveillance (confirmed at the 2014 Black Lives Matter protests in Chicago, the 2017 Standing Rock protests in North Dakota, multiple Hong Kong demonstrations).
Who uses it, and against whom
In the US, deployed by FBI, DEA, USMS, ICE, US Secret Service, and at least 75 state and local law enforcement agencies that ACLU has documented through public records. The Department of Justice required a warrant for federal use in 2015 (a major shift from prior pen-register-only authorization). State and local rules vary widely; many local agencies still deploy under non-disclosure agreements signed with Harris that prohibit revealing the technology in court, leading prosecutors to drop charges rather than disclose. Internationally: every nation-state SIGINT service has equivalents, plus deployments by authoritarian governments documented in Belarus, Russia, Saudi Arabia, China.
What you can change today
Three protections of decreasing convenience. First, in iPhone Settings (Cellular, Cellular Data Options) and on Android (Settings, Network, Preferred Network Type), disable 2G fallback so the phone refuses to connect to the unencrypted 2G mode that most cell-site simulators exploit for content interception. Second, for sensitive meetings or protest attendance, leave the phone at home or run it in airplane mode for the duration; cell-site simulators capture devices that radio, not devices that don’t. Third, for higher-stakes operations, use a faraday bag for transit and only power the device on at the destination, ideally on Wi-Fi only with cellular fully disabled.