A Black Box is an electronic circuit (normally a resistor and capacitor placed in parallel) that can be placed on a phone line to provide a free phone call on legacy telephone systems. In order for a call to be free for the originating and receiving station, both have a have Black Box or other method implemented to avoid call charging. The box was a popular method used by telephone phreakers from the 1960s to 1980s. An alternative use of the Black Box is to incorporate them into the incoming modems of computers that were used to run BBS’s (bulletin board systems) in the 1980s and early 1990s.

What is Phreaking?

Phreaking is a term used to describe the act of using an electronic device or computer to deceive a phone system. Typically, phreaking is used to allow the individual to make free phone calls or have them charged to another individual’s account. As telecommunications networks have become computerized and networked, the act of phreaking has become closely associated with network hacking. Black Boxes were one of the methods used by early “phreakers” to make free phone calls or conduct prank calls before the adoption of modern electronic switching systems (ESS’s) by the telecommunications industry. Today, Black Boxes can still find use on legacy phone systems which have not made the shift to digitally switched networks.

How Legacy Electromechanical Telephone Exchanges Worked

Phone systems which used Common Channel Signaling (CCS), or electromechanical exchanges, typically use two relays to

detect the line status of a called phone. For the legacy British Strowger phone exchanges, the two relays were referred to as the F and D relay found in the final selector. Although the labeling of these two relays may differ for the respective phone exchange, they will be used to describe the function of how a Black Box works.

The F relay is used to detect the ring trip on a phone. Once activated, the called phone is switched from the ringing supply to the audio path. The D relay is then used to start and stop the metering of the phone call. Phreakers use this knowledge to leverage Black Boxes to activate the F relay but not the D.

On the legacy phone systems, the ringing supply consisted of an AC (alternating current) signal of approximately 75 volts that is superimposed on a 48 volt DC signal. The telephone ringer is connected in series with a capacitor that would allow the AC to pass through and sounding the ringing when “on-hook” but blocking the DC current at the same time. The F relay coil is designed to not be sensitive to AC and only activates when receiving DC. Once the phone is “answered” or taken “off-hook,” it appears to be a low-value resistor to the phone exchange that activates the F relay. The F relay remains activated even after the phone is subsequently placed “on-hook.”

How Do Black Boxes Work?

When Black Boxes were in wide-spread use, they would operate by taking advantage of the “feature” found in most phone exchanges by only metering phone calls when the called phone is “off-hook.” Metering is suspended in the called phone was placed “on-hook.” When used properly, a Black Box makes a phone which is “off-hook” appear to be “on-hook” electrically. As a result, the telephone exchange is being tricked into believing the called phone is on-hook.

Basic Black Box Design

The basic Black Box is made of a resistor and capacitor wired in parallel. The device is connected in series with the telephone. When in wide-spread use, the installation of a Black Box in a phone was straightforward due to the phones at the time using screw terminals.  This allowed installation in a phone in between the appropriate terminals in place of the link. Today, soldering may be required to install a Black Box on a phone.

The Black Box resistor’s purpose is to supply enough direct current (DC) to trip the F relay (or equivalent) and operate the carbon transmitter. The value had to be sufficient to reduce the DC to a point where the D relay (or equivalent) would fail to trip when the phone was “off-hook.” The classic resistance value is 10 kΩ, although many phreakers experiment with the value based on the specific voltages required due to the length of the local phone loop and type of telephone exchange equipment. If the value of the resistance was too high, the F relay would fail to activate. Too low, and the D relay would activate when the phone was “off-hook” and result in charges.

The Black Box capacitor is used to provide a low-impedance path for the alternating current (AC) audio signal. The typical value of a Black Box capacitor ranges between 0.1 μF to 1 μF. Newer or more refined black boxes can insert a diode in parallel with the resistor to give a larger DC pulse for activating the F relay as compared to only using a resistor. Sometimes, the resistor can be fully replaced with a 36V Zener diode that would regulate the voltage drop across the phone to a point where the D relay would fail to activate independent of the local phone loop.

Advanced Black Box Design

Advanced Black Box designs include a 9 volt battery to power the phone. This avoids the issue of attempting to draw enough current to power the phone beneath the threshold required to avoid starting the phone meter. The battery is used to feed power to the phone via an inductor. The phone line is then connector via a capacitor to the terminals of the inductor. The inductor has to be of a high enough inductance to allow AC audio signals to travel to and from the phone line while allowing the battery current to pass through to power the phone. The diode is connected across the phone line. In this setup, the diode is activated via manual push-button prior to picking up the phone to avoid phone metering from being turned on or activated.

Some legacy phone exchanges reverse the line polarity of the calling phone when the D relay was active. Phreakers can use this to test the effectiveness of a Black Box. An alternative method to test the device is to use a payphone if a house line was not available for testing (or too risky to use for testing).

Modern Countermeasures Against Black Boxes

Today, most digital phone exchanges are made to prevent Black Boxes from working. Many exchanges do not suspend metering when a called phone is placed in the “on-hook” condition. Others will terminate a call whenever a called phone is placed “on-hook” for a given amount of time.

What Is a Blue Box?

A Blue Box is another electronic device used by phreakers which creates the same tone as used by a phone operator’s dialing console for long-distance call switching. The device first saw use in the 1960s and 1970s and allowed individuals to route phone calls through in-band signal emulation used to control long distance call switching. The boxes were primarily used to place free phone calls. Today, the Blue Box is primarily useful (although illegal in most countries) on phone networks that do not use digital switching.

What is a Red Box?

A Red Box is another phreaking device that was used to create tones that mimic the sound of inserting coins in a pay phone. If used correctly, the phone system would think that actual money was inserted into the phone allowing an individual to make phone calls for free. Any electronic device capable of playing back tones could be used in a Red Box (such as tone dialers, MP3 players, or even audio-recording greeting cards).