For an exchange which can receive pulse-mode signals, if the dial rate of the signal on the line is consistent with the make-break-ratio of the signal, the exchange will accept this pulse-mode signal. Referring to FIG. 1, HKSW is a pulse state of a telephone hook switch and DP represents a pulse-mode signal which is sent out from the telephone to the exchange. The symbol Tm and Tb in FIG. 1 represent the make and the break time respectively. The symbol Tf is the dial time so that 1/Tf represents the dial rate. The output numbers represented by pulse groups of DP in FIG. 1 are 3 and 2. HKSW is "low" when the telephone is in an off-hook operational mode, i.e. when the receiver of the telephone is lifted up and "high" when the telephone is in an on-hook operational mode, i.e. when the receiver is put back. In general, the operation rate of HKSW obtained by manipulating the hook switch is different from that of DP generated by dialing a dial or a keypad. However, as shown in FIG. 2, the pulse state of HKSW can be made consistent with the pulse state of DP if the user tries to reach the operation state of the hook switch by way of lifting up and putting back the receiver or intermittently pressing the hook switch in a certain rate. That is, the signal generated by the hook switch can substitute for the signal generated by the dial or the keypad and be accepted by the exchange. Then, the telephone lock-control system or code-identifying system will possibly be void or null. For example, in a telephone lock-control system which disallows the signals representing telephone numbers beginning with 0 to be accepted by an exchange, the pulse groups representing the numbers after 0 will not be sent out by the telephone if the telephone detects a pulse group representing the number 0 generated by a dial or a keypad in the beginning. However, if the pulse group representing the number 0 is not generated by the dial or the keypad but by the hook switch, the pulse groups representing the numbers after 0 will be allowed to be sent out and the signal representing the telephone number will be accepted by the exchange. By this way, the telephone lock-control system is rendered ineffective.