First of all to test for bugs, you
need a VOM (Multimeter) the higher
the impedance the better (a Digital
with FET circuitry or a Vacuum Tube
Volt Meter is the best).
First disconnect the phone line(s) AT BOTH ENDS. Undo the phone instrument and hook it up to the entry point of the phone line from the outside world (Ma Bell does not like you cut her off completely.) The scheme is the physi- cally isolate your house, apartment, etc from the outside world. But before you do this measure the line voltage (It should be approximately 48 Volts).
Now with the wires disconnected at both ee set your resistance scale to a high reading and measure the resistance of the phone line, it should be very high on the order of million ohms or more, this is the normal condition, since you are measuring the resistance of an open circuit. If it is much less, say 50-100Kohms then you a device on the line that does not belong there, probably a parall disconnected wire and go to the other end and measure the resistance of this. This resistance should be about one ohm or two at the most in a big house with a lot of phones. If it is more, then you probably have a series bug.
If in the first case, taking parallel measurements using a meter (not LED/ LCD) and you notice a "kick" in the needle, you probably have a line tap
Now if you also make a measurement with the wire end twisted together and you notice the resistance reads about 1-2kohms, then you may have a drop-out relay. A drop-out relay is a relay that senses a phone going off hook, and signals a tape recorder to start record- ing.
Another test to do with the phones still hooked up to the outside world, on hook voltage is about 48 Volts and off hook is about 6-10 Volts. Any other con- ditions may mean telephone surveillance.
If you use a Wide Range Audio frequency generator and call you house, apartment, etc. from another phone and sweep up and down the spectrum, and you notice the phone answers itself somewhere in the sweep you probably have an infinity transmitter on your line. There are sometimes telco sweep tones available for dialup, check your exchanges 99xx and 00xx numbers.
The above information tells nothing aut TELCO taps at the central office, or anywhere else along the line, but this information may tell you that your wife, girlfriend(boyfriend), or business associate may be monitoring your phone activities.
An Infinity transmitter, is a neat device It allows you to call the bugged place and it shuts off the ringer and defeats the switchhook, so the mouthpiece now becomes a room bug. It was orginally sold from the travelling business man to make sure his wife was safe at home not being attacked (or fucking her boyfriend behind his back).