What are telephone wiretaps?

When discussing the various ways in which information can be extracted from an area, most people use the term “bug.” This is an all inclusive term which encompasses everything placed on the telephone lines. In reality, these are two different types of attacks on the telephone lines with two different names. How should they be described?

In the simplest description, a “bug” is generally an externally powered device which radiates (transmits) a signal containing intelligence, whereas a “tap” is an attachment normally made to a pair of lines for the purpose of recording or monitoring intelligence, be it audio, video or data.

In describing a tap, the first thing to come to mind is the telephone tap. As glamorized on television, it is thought of as a “drop-in” microphone that transmits the conversation taking place within a room. In reality, this is a bug and not a tap. Also, we see the eavesdropper sitting in an office or basement with headphones listening to conversations on the telephone. This is a fairly true representation of what happens but there are more ways of “tapping.” They can range from the crude attack of hooking a sound powered (telephone repairman) handset directly to the lines, to the more sophisticated method of using a high impedance device between the lines and a recorder or amplifier to using an inductive device in which the telephone or lines are not physically altered.

If the telephone instrument has not been modified, then the only time it can be monitored is when the telephone is in actual use. However, with any one of a number of simple to sophisticated attacks, it is possible to “listen in” at all times to everything that is being said in the vicinity of the telephone. Eavesdroppers can install in-line “series” or “parallel” transmitters on the telephone line in which no external power supplies are necessary - power is taken from the telephone lines. No antenna or microphone is required. These devices can be placed at any point on the telephone line from the telephone to the nearest switching equipment. They can be as large as the cover on a fountain pen or they can be as small as a dime. Operating frequencies can range from as low as 30 MHz to over 2000 MHz. That is not to say this is the only range of frequencies in which they will be found, only that it easier to build the devices to operate within these ranges.

Other attacks that can be made on the telephone include the manipulation (bending) of contacts on the hook-switch, the addition of jumpers in the telephone or the installation of resistors or capacitors. These attacks will allow your conversations to be monitored, whether the telephone is on hook of not. The items discussed above are not the only attacks possible to a telephone system. They are just a representative sampling.