SS7 (Signalling System No. 7) is a signalling protocol for telephone networks in use since the 1980s. It handles call setup, SMS delivery, and roaming. SS7 was designed without security mechanisms because it originally ran only between trusted carriers. Today, access via IP gateways enables attacks such as location tracking, SMS interception, and call redirection. You can barely mitigate SS7 vulnerabilities on the endpoint side directly. In an ISMS, SS7 is relevant when SMS-based authentication is used — switching to app-based tokens is advisable.