A method of verifying the validity of a message received by a telecommunications terminal (8) having a processor (30) and which is operable in a boot mode and a runtime mode is disclosed. In the embodiments the message is a SIM unlock message, for removing or modifying a restriction of the types of subscriber identity module (SIM) with which the mobile terminal may be used. In response to reception of the SIM unlock message when the terminal (1) is in the runtime mode, the processor 30 of the terminal (1) causes the terminal to enter the boot mode and verify the validity of the message during the boot mode. Because the validity of the message is checked during the boot mode, the check can be performed with greater security.