An exemplary circuit for preventing unauthorized access to a phone includes three transistors, a zener diode, and a rectifier comprising two input terminals for receiving an AC voltage from the telecommunication office, a negative output terminal connected to the phone via a first resistor, and a positive output terminal connected to the phone. The anode and the cathode of the zener diode are respectively connected to the positive output terminal and the base of the first transistor. The collectors of the first and third transistors are connected to the base of the second transistor, which is connected to the positive and negative output terminals respectively via two capacitors. The collectors of the first and second transistors are connected to the positive output terminal respectively via a second resistor and a buzzer. Emitters of the three transistors are connected to the negative output terminal of the rectifier.