Silent SMS (Stealth Ping)
An invisible text message that produces no notification on the recipient's phone but forces the device to register with a cell tower, revealing its approximate location.
Also known as: Class 0 SMS, flash SMS, stealth ping, silent text surveillance
Last reviewed: 10 June 2026
A silent SMS, also known as a Class 0 message or flash SMS, is a standard-compliant message type that most phones process without displaying a notification or storing in the inbox. The sending device receives delivery confirmation, and the recipient's device is forced to register with its nearest cell tower in the process. By correlating delivery confirmations from multiple towers, the sender can estimate the recipient's location.
Silent SMS has been used by law enforcement for targeted surveillance and is reportedly used by intelligence agencies in several countries. Although it has legitimate investigative applications, the technique is also available to anyone with access to an SMS gateway that permits Class 0 messages, and has been documented in corporate espionage and stalkerware deployments.
There is limited practical defence for typical consumers. Some Android security tools can detect Class 0 messages and alert the user. Keeping your phone on 4G or 5G rather than falling back to 2G reduces location accuracy. For high-risk individuals — journalists, activists, executives — using a dedicated device with a separate SIM for sensitive activities is a partial mitigation.