Telephone System/Line Analysis
Wired and wireless transmitters must be physically connected to the line before they will do any good. Once a wireless tap is connected to the line, it can transmit all conversations over a limited reception range. Wired taps, on the other hand, need no power source, but a wire must run from the line to the listener or to a transmitter.
An 'Infinity Transmitter' or 'Harmonica Bug' must be installed inside the phone. When someone calls the tapped phone it rings, blows a whistle over the line, and the transmitter picks up the phone via a relay. The mike on the phone is activated so that the caller can hear all of the conversations in the room.
The telephone line analyzing equipment used during the sweep (photo above) is capable of determining illegal taps, defective phones, room listening devices, splices, VLF devices, infinity triggered devices, and selectively targeted phones in situations where a tap has been placed in a wire closet or a telephone room.
A signal is generated from your phone right to the Bell Canada Phone Centre (or the main phone centre of your particular State or Province). The purpose is to evaluate the current in the line and to discover if any device has been placed on the line that might be causing a drop in the current.