Logistics of wiretapping

"When telephone exchanges were mechanical, a tap had to be installed by technicians, linking circuits together to route the audio signal from the call. Now that many exchanges have been converted to digital technology tapping is far simpler and can be ordered remotely by computer. Telephone services provided by cable TV companies also use digital switching technology. If the tap is implemented at a digital switch, the switching computer simply copies the digitzed bits that represent the phone conversation to a second line and it is impossible to tell whether a line is being tapped. Even a well designed tap installed on a phone wire can be difficult to detect. The noises that some people believe to be telephone taps are simply crosstalk created by the coupling of signals from other phone lines.

Data on the calling and called number, time of call and duration, will generally be collected automatically on all calls and stored for later use by the billing department of the phone company. This data can be accessed by security services, often with fewer legal restrictions than for a tap. This information used to be collected using special equipment known as pen registers and trap and trace devices and U.S. law still refers to it under those names. Today, a list of all calls to a specific number can be obtained by sorting billing records. A telephone tap during which only the call information is recorded but not the contents of the phone calls themselves, is called a Pen Register tap."

http://en.wikipedia.org/wiki/Wiretap