Telephone Instrument Attacks
Perhaps the most insidious and least recognized method of eavesdropping on room conversations is by telephone bugging or a hookswitch bypass.
The hookswitch bypass is a technique for room bugging which enables a telephone instrument to transmit room audio while the handset is in the on-hook position. This attack includes various methods by which the telephone instrument is modified or rewired to convert it to a continual listening device, even when it is hung up.
This simple technique usually allows the eavesdropper to intercept both telephone and room conversations without having to be concerned about hiding devices, running wires, or changing batteries within the target area.
There are several reasons why the telephone, as a listening device, is preferable to other bugging techniques:
The telephone, with up to three possible microphones or transducers (the transmitter, typically a carbon or electret microphone, the magnetic earphone receiver, and the ringer circuit), is usually at an optimum location to accomplish eavesdropping in a target area. An additional microphone could also be installed within the telephone instrument by an eavesdropper.
The telephone system provides conductors to carry the acquired audio to a listening post. Because the microphones and conductors are inherent to the telephone, there are no concealment requirements.
No power is required in the target area, since the power used is either telephone system power or provided from the listening post. Therefore, battery replacement is not necessary. Eavesdropping on telephone conversations is indeed a threat; however, far more critical information is frequently uttered after the telephone call has ended. this is particularly true in an office with the telephone on the desk or credenza, which is typically the center of most conversations.
A hookswitch bypass is the perfect eavesdropping method for gathering the maximum amount of intelligence.