Dialling (telephony)

Last updated

Dialling, (dialing in US English), is the action of initiating a telephone call by operating the rotary dial or the telephone keypad of a telephone.


Rotary dial

Prior to Strowgers invention of SXS, rotary switch in 1891, telephone connections involved cranking a handle to generate a voltage that operated a bell on the remote operator's board; the operator would then on the line to the subscriber and speak to them, they would then raise a voltage on the recipients phone, alerting them of an incoming call. When this was acknowledged, 88 operator would patch the two subscribers together using a lead terminating in a jack plug. [1] [2]

Under Strowger's system that was first introduced commercially in La Porte, Indiana the number was dialled using two telegraph keys. The subscriber tapped eight times on one, then three times on the second to 'dial' the number 83. When this was introduced into Albion, New York in 1899 – the keys had been replaced by a rotary dial. [2]

National adoption

GPO 332 Telephone with letter codes GPO 332 Telephone.JPG
GPO 332 Telephone with letter codes

From 1912, the British General Post Office, which also operated the British telephone system, installed several automatic telephone exchanges from several vendors in trials at Darlington on 10 October 1914 (rotary system), Fleetwood (relay exchange from Sweden), Grimsby (Siemens), Hereford (Lorimer) and Leeds (Strowger). [3] The BPO selected the Strowger switches for small and medium cities and towns. However, the selection of switching systems for London and other large cities was not decided until the 1920s, when the Director telephone system was adopted. The Director systems used SXS switches for destination routing and number translation facilities similar to the register used in common-control exchanges. Using similar equipment as in the rest of the network was deemed beneficial and the equipment could be manufactured in Britain.

Touch tone dial

DTMF keypad layout DTMF keypad layout.svg
DTMF keypad layout

Introduced to the public in 1963 by AT&T, Touch-Tone dialing greatly shortened the time of initiating a telephone call. It also enabled direct signaling from a telephone across the long-distance network using audio-frequency tones, which was impossible with the rotary dials that generated digital direct current pulses that had to be decoded by the local central office.

The touch tone key pad has sixteen keys laid out in a four-by-four matrix. Each key produces a combination of two audible tone frequencies, determined by their position on the pad. Each column and row has a distinct frequency assigned, thus generating a total of sixteen dual-tone multi-frequency (DTMF) signals. Early keypads only used the keys with the digits 1 through 9 and 0. The extra keys were intended for computer data entry, business functions, and military applications. By 1967, the keys with the asterisk (*) and hash (#) were added on all subscriber telephone sets. The keys A, B, C, D were used as priority signals (FO, F, I, and P) in the Automatic Voice Network (AUTOVON) of the US military.

Initial pushbutton designs employed mechanical switches, so that each button activated certain combinations of capacitors and inductors of an oscillator, while later versions used semiconductor logic chips to synthesize the frequencies. The tones are decoded by the receiver to determine the keys pressed by the user.

DTMF keypad frequencies (with sound clips)
1209 Hz1336 Hz1477 Hz1633 Hz
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D

For example, the key 1 produces a superimposition of tones of 697 and 1209 hertz (Hz).

Related Research Articles

Dual-tone multi-frequency signaling

Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed in the Bell System in the United States, and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. DTMF is standardized as ITU-T Recommendation Q.23. It is also known in the UK as MF4.

Rotary dial Component that allows dialing numbers

A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number to a telephone exchange.

Telephone switchboard

Throughout the 20th century, telephone switchboards were devices used to connect circuits of telephones to establish telephone calls between users and/or other switchboards. The switchboard was an essential component of a manual telephone exchange, and was operated by switchboard operators who used electrical cords or switches to establish the connections.

Telephone Telecommunications device

A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound, typically and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. The term is derived from Greek: τῆλε and φωνή, together meaning distant voice. A common short form of the term is phone, which came into use almost immediately after the first patent was issued.

Pulse dialing

Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten Arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. Historically, the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing.

In telephony, ringdown is a method of signaling an operator in which telephone ringing current is sent over the line to operate a lamp or cause the operation of a self-locking relay known as a drop.

Phreaking is a slang term coined to describe the activity of a culture of people who study, experiment with, or explore telecommunication systems, such as equipment and systems connected to public telephone networks. The term phreak is a sensational spelling of the word freak with the ph- from phone, and may also refer to the use of various audio frequencies to manipulate a phone system. Phreak, phreaker, or phone phreak are names used for and by individuals who participate in phreaking.

Plain old telephone service (POTS), or plain ordinary telephone service, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1876 until 1988 in the United States when the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) was introduced, followed by cellular telephone systems, and voice over IP (VoIP). POTS remains the basic form of residential and small business service connection to the telephone network in many parts of the world. The term reflects the technology that has been available since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).

A dial tone is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is working and is ready to initiate a telephone call. The tone stops when the first dialed digit is recognized. If no digits are forthcoming, the partial dial procedure is invoked, often eliciting a special information tone and an intercept message, followed by the off-hook tone, requiring the caller to hang up and redial.

Blue box Electronic device used to illegally place free long-distance telephone calls

A blue box is an electronic device that generates the in-band signaling tones formerly generated by telephone operator consoles to control telephone switches. Developed during the 1960s, blue boxes allowed private individuals to control long-distance call routing and to bypass the toll-collection mechanisms of telephone companies, enabling the user to place free long-distance telephone calls on national and international circuits.

Strowger switch Electromechanical telephone switch

The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of its operational characteristics it is also known as a step-by-step (SXS) switch.

Telephone keypad

A telephone keypad is the keypad installed on a push-button telephone or similar telecommunication device for dialing a telephone number. It was standardized when the dual-tone multi-frequency signaling (DTMF) system was developed in the Bell System in the United States in the 1960s that replaced rotary dialing originally developed in electromechanical switching systems. Because of the installed abundance of rotary dial equipment well into the 1990s, many telephone keypads were also designed to produce loop-disconnect pulses electronically, and some could be optionally switched to produce either DTMF or pulses.

In telephony, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after World War II. It uses a combination of audible tones for address transport and supervision signaling on trunk lines between central offices. The signaling is sent in-band over the same channel as the bearer channel used for voice traffic.

Automatic Electric Company (AE) was the largest of the manufacturing units of the Automatic Electric Group. It was a telephone equipment supplier for independent telephone companies in North America, and also had a worldwide presence. With its line of automatic telephone exchanges, it was also a long-term supplier of switching equipment to the Bell System, starting in 1919.

Director telephone system

The director telephone system was a development of the Strowger or step-by-step (SXS) switching system used in London and five other large cities in the UK from the 1920s to the 1970s.

Model 500 telephone

The Western Electric model 500 telephone series was the standard domestic desk telephone set issued by the Bell System in North America from 1950 through the 1984 Bell System divestiture. Millions of model 500-series phones were produced and were present in most homes in North America. Many are still in use today because of their durability and ample availability. Its modular construction compared to previous types simplified manufacture and repair, and facilitated a large number of variants with added features.

Tip and ring Two conductors or sides of a telephone line

Tip and ring are the names of the two conductors or sides of a telephone line. The terms originate in reference to the telephone plugs used for connecting telephone calls in manual switchboards. One side of the line is connected to the metal tip of the plug, and the second is connected to a metal ring behind the tip, separated and insulated from the tip by a non-conducting material. When inserted into a jack, the plug's tip conductor connects first, followed by the ring conductor. In many European countries tip and ring are referred to as the A and B wires.

Push-button telephone

The push-button telephone is a telephone that has buttons or keys for dialing a telephone number, in contrast to having a rotary dial as in earlier telephone instruments.

Telephone exchange Interconnects telephones for calls

A telephone exchange or telephone switch is a telecommunications system used in the public switched telephone network or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.

Panel switch

The Panel Machine Switching System is an early type of automatic telephone exchange for urban service, introduced in the Bell System in the 1920s. It was developed by Western Electric Laboratories, the forerunner of Bell Labs, in the U.S., in parallel with the Rotary system at International Western Electric in Belgium before World War I. Both systems had many features in common.


  1. Calvert, J. B. (2003). "Basic Telephones" . Retrieved 11 August 2016.
  2. 1 2 Rochester's Remarkable Past Chapter 11. By Donovan A. Shilling, Pancoast Publishing, 2011
  3. Events in Telecommunications History – 1927 BT Archives