| 摘要 |
A detector for a "Black Box" toll fraud is connected to circuits at the called party's central office and comprises a relay connected in series with the called subscriber's tip (T) lead, a fraudulent call register or indicator, and a normally open time delay switch with a timer actuated by a ringing signal on the ring (R) lead, which switch closes after a predetermined delay interval (ten seconds) to cause energizing of the register. The flow of direct current in the T lead incident to the called party going off-hook energizes the relay and deactivates the switch. The control (C) lead of the central office subscriber circuit is grounded by the off-hook state of the calling party's telephone and is connected to the register by closing of the delay switch, the indication of a fraudulent call being made by the register on removal of ground from the C lead, i.e., when the calling party goes on-hook. A terminal for connecting to the starting mechanism of a recorder is also provided in parallel with the time delay switch for turning the recorder on so that a permanent record of the fraudulent call may be made. In a modified form of the invention, means are provided to prevent indication of a fraudulent call by the register in the rare event a calling party does not go on-hook within the period of the delay interval following return of the called party's telephone to the on-hook position.
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