This is my first post but i've been following ATS for a some time now. I really had to give my point of view on this matter as NMT- and GSM-networks
were big part of my vocational examination so i dare to say that i have some knowledge of what i'm talking about..
I've summarised here a quick cut and paste from this webpage
as i'm too lazy and my grammar
sucks.. Please read the webpage for more thorough explanation.
In the U.S., telecommunications carriers are required by law to cooperate in the interception of communications for law enforcement purposes under the
terms of Communications Assistance for Law Enforcement Act (CALEA)
. Taps must be secret and
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. Public switched
telephone network and 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 digitized bits that represent the phone conversation to a second line and it is impossible to tell
whether a line is being tapped. 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.
* "Recording the conversation" - the person making/receiving the call records the conversation using a coil tap ('telephone pickup coil') attached
to the ear-piece, or they fit an in-line tap with a recording output. Both of these are easily available through electrical shops.
* "Direct line tap" - this is what the state used to do via the telephone exchange. But unofficial tapping, where the user's line is physically
tapped near the house, is also possible. The tap can either involve a direct electrical connection to the line, or an induction coil. An induction
coil is usually placed underneath the base of a telephone or on the back of a telephone handset to pick up the signal inductively. With a direct
connection, there will be some drop in signal levels because of the loss of power from the line, and it may also generate noise on the line. A well
designed induction tap does not drain voltage or current from the line because it isn't physically connected to the phone line. Direct taps sometimes
require regular maintenance, either to change tapes or replace batteries, which may give away their presence.
* "Radio tap" - this is like a bug that fits on the telephone line. The state does not normally do this because they have access via the telephone
exchange, though certain organizations exempt from the common framework of law applying to citizens may use devices like this. It can be fitted to one
phone inside the house, or outside on the phone line. It may produce noise (there might even be signal feedback on the monitored line on poorly made
equipment) to inadvertently alert the caller.
Many of you probably know this already but to those that don't:
It is also possible to get greater resolution of a phone's location
by combining information from a number of cells surrounding the location,
which cells routinely communicate and measuring the timing advance, a correction for the speed of light in the Global System for Mobile Communications