Phone taps can be used for audio surveillance in a number of ways. They can be used to transmit a conversation to a remote location, where someone else can listen to it; they can tap a phone to record a conversation as it occurs (with the option to manually or automatically record); or they can simultaneously transmit and record the content of a call.
Some taps can be placed either directly in the ear piece (“coil taps”) or somewhere on the telephone line. For those that go on the line, there is also a basic difference between “series” phone taps and “parallel” phone taps.
A series tap, which is the more common, inserts the bugging device into the existing wire, so the existing phone line must be temporarily cut to install the device.
The effect is to intercept the signal flowing “downstream” from the main exchange. That means a series tap must go somewhere between the incoming telephone line and the phone that is the target of your operation. In a home residence, if all the phones are on the same line, then a series tap will be able to target all of them provided it is placed “above” each of them on the incoming line.
If there are different lines, it will only target the phone or phones that are downstream from the tap on that particular line. A parallel phone tap is installed across the existing wires without having to cut them. Its wires attach to the same terminal points that anchor the existing wires.
This, by contrast, can target every telephone on the line no matter where it is placed on the line. A series tap can transmit a conversation to a receiver approximately 100 to 500 feet away. They are typically “parasitic” devices, which means they can suck enough power directly from the phone line without the need for any batteries or external power source.
The range for a parallel tap transmission is greater, with a range of between 200 and 1000 feet. It does require an external power source. These phone taps are both readily available from a local hardware or electronics shop, and retail for anywhere between $30 to several hundred dollars depending on the recording and transmitting capabilities you are after.
Another form of tap that is worth consideration is called a “modular jack” transmitter, and its beauty is in its ability to remain disguised in plain view. It simply plugs into the existing phone jack and looks like nothing other than an additional splitter. It can receive at a range over 1000 feet with a decent receiver. You can order it online fully assembled for about $75.