A bit sequence, which is contained in a signalling message and which is known to a network unit and to a communications terminal which receives the signalling message from the network unit, informs the communications terminal that a test value is contained in a signalling message. The test value received by the communications terminal is compared with a test value computed by the communications terminal, and the communications terminal defines a signalling message as being unmodified only in the event that the bit sequence contained in a signalling message has been received and the comparison of both test values yields a positive result.