A coin fraud prevention system which includes a coin telephone having a voice transmitter, comprising a coin tone sensing arrangement for sensing coin tones, coupled to the voice transmitter of the telephone; and a coin telephone disabling arrangement in the coin telephone coupled to an output of the coin tone sensing arrangement for disabling the coin telephone in response to sensing of the coin tones from the transmitter; wherein further the coin tones include at least one or two coin tone frequencies, and the system has a bandpass filter having at least one pass band corresponding to the tone frequencies, and a tone detector coupled to the bandpass filter for generating a coin telephone disabling signal in response to receipt of the tone frequencies, wherein the disabling signal is connected to the coin telephone disabling arrangement, so as to prevent dialing of the call.

A telephone station (1) having a handset (14) can be connected to a telephone exchange (5) by means of first and second switches (6, 7) by way of two lines (2, 3). When the switches (6, 7) are closed the telephone station (1) can be supplied with electrical energy from the telephone exchange (5) by way of the lines (2, 3). Within the telephone station (1) the first switch (6), a first coil (8), a circuit member (10), a second coil (9) and the second switch (7) are electrically connected in series. The switches (6, 7) are actuable by the handset (14) of the telephone station (1). The two coils (8, 9) are wound in opposite directions around a body (12) of magnetic material. There are also provided a measuring coil (11) and a measuring circuit (13) for producing an alternating magnetic field in the body (12) and for measuring the time derivation dB/dt of the magnetic induction B(t) which occurs in the body (12) and for producing a signal for opening the switches (6, 7) if the measured derivation dB/dt fulfills predetermined criteria. The coils (8, 9), the magnetic body (12), the measuring coil (11) and the measuring circuit (13) serve to prevent attempts at fraud.

Pin fraud of battery reversal pulse metered sophisticated coin telephones is detected, an announcement of the detected fraud made, and the fraud is stopped if the detected fraud continues by the method and apparatus of the invention. By carefully monitoring within the switch office the loop current that is flowing in the 48 volt battery circuit, especially in the more positive terminal portion of the 48 volt battery circuit, loss of current to a fraud ground at a remote coin telephone can be detected. Once a fraud ground is detected, an announcement is made to at least the calling station where the pin fraud was detected, which says in effect "The pay telephone you are using is charging improperly for the service provided. If such improper charging continues the present call will be discontinued." A convenient location in the switch office and an inexpensive component configuration make the addition of this pin fraud detection, announcement and prevention apparatus very cost effective.

Pin fraud of battery reversal pulse metered sophisticated coin telephones is detected and prevented by the method and apparatus of the invention. By carefully monitoring within the switch office the loop current that is flowing in the 48 volt battery circuit, especially in the more positive terminal portion of the 48 volt battery circuit, loss of current to a fraud ground at a remote coin telephone can be detected. Once a fraud ground is detected, the call is terminated. The current detector of this method and apparatus can also be used to terminate a call for the normal on-hook break of the loop current. A convenient location in the switch office and an inexpensive component configuration make the addition of this pin fraud detection and prevention apparatus very cost effective.