The invention relates to a method and to an arrangement for identifying an unauthorized user (A2) of a mobile telephone (M) that includes a terminal identity (m) and a chosen security code (PIN1). When the authorized owner (A1) has placed his/her authorized SIM-card (SIM1) in the mobile telephone (M) and has entered a security code (PIN1) into the mobile telephone (M), the mobile telephone ties a SIM-identity (s1) of the SIM-card (SIM1) to the terminal identity (m). If the authorized user (A1) loses his/her mobile telephone (M), the subscription is blocked. An unauthorized user (A2) purchases a subscription and is issued with a second SIM-card (SIM2), which he/she places in the mobile telephone (M). When the unauthorized user (A2) enters a security code (PIN2') that does not match the chosen security code (PIN1), the mobile telephone (M) compares the SIM-identity (s2) of the second SIM-card (SIM2) with the terminal identity (m). Since these identities (s2, m) do not match one another, the mobile telephone (M) automatically sends the identities (s2, m) to a service node (F), which confirms receipt of the identities by sending a response signal (RES) to the mobile telephone (M). The service node (F) is able to obtain personal user information (A2-info) from the database (D2) of the operator (P2) relating to the unauthorized user (A2) of the mobile telephone (M), via the SIM-identity (s2).
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