Push-button telephone
A push-button telephone, commonly known as a touch-tone phone, is a type of telephone that employs a numeric keypad with push buttons to initiate calls by generating dual-tone multi-frequency (DTMF) audio signals, which are transmitted over the phone line to connect the call, supplanting the older rotary dial system that used electrical pulses.[1] Developed by engineers at Bell Labs under the leadership of human factors specialist John E. Karlin, the technology underwent extensive testing of keypad layouts before settling on the standard 3x3 numeric grid with 1 at the top left and 0 below, a design that later became an international standard for various electronic interfaces.[2] The first commercial model, the Western Electric 1500, featured 10 buttons and was introduced by the Bell System on November 18, 1963, in Carnegie and Greensburg, Pennsylvania, marking the debut of electronic push-button dialing for public use.[1][3] This innovation enabled significantly faster dialing speeds, reducing the time required to dial a typical seven-digit number from about 10 seconds to 2 seconds—and facilitated direct long-distance calling without operator assistance, while also paving the way for advanced features like automated services and computer interfaces through the addition of * and # keys in later models.[2][4] By the 1980s, push-button telephones had achieved widespread adoption in U.S. households, effectively phasing out rotary dials and transforming everyday telephony into a more efficient and versatile system.[1]History
Early Experiments and Development
The origins of push-button telephones can be traced to late 19th-century efforts to automate telephone dialing, beginning with concepts like the rotary dial patented by Almon Brown Strowger in 1891, which sought to replace manual operator assistance with mechanical switching and laid the groundwork for non-rotary alternatives in the early 20th century. Early experiments in push-button mechanisms emerged in the 1930s, exemplified by the Dialaphone developed by James Kilburg of the James Kilburg Corporation in San Mateo, California, which used push-buttons to select pre-punched codes on cards for rapid automatic dialing, though it remained limited to niche applications.[5] In 1941, Western Electric, the manufacturing arm of the Bell System, began experimenting with tone-based dialing systems as precursors to modern push-button technology, employing mechanically activated reeds to generate two distinct audible tones for each of the ten digits; these efforts were temporarily shelved during World War II due to resource constraints.[6] By the late 1940s, Bell Laboratories resumed and advanced this research, conducting field tests of prototype telephones that utilized audible tones for signaling over existing telephone lines, including a 1948 trial in Media, Pennsylvania, involving a 302-style set with two rows of five keys operating on two-out-of-five frequencies connected to a No. 5 crossbar switch for 35 households, which revealed initial instability issues but demonstrated feasibility.[6] These prototypes, developed under the leadership of human factors specialist John E. Karlin at Bell Labs, who oversaw extensive testing of various keypad layouts before adopting the standard 3x3 grid, marked a shift toward multifrequency signaling methods.[6][2] During the 1950s, parallel research occurred outside the Bell System, notably by the Dutch company Philips, which developed a direct current (DC)-based push-button system for private branch exchanges (PBX), as seen in the UB-49 PBX introduced around 1959, using a special GDK push-button dial with semiconductor diodes to produce unique DC signals for each digit without relying on tones.[7] Bell Labs continued refining tone prototypes, including circa-1955 tests of a 500-type set with key-pulse dialing and pre-1958 trials of a 10-button model using one-of-ten frequencies in locations like Americus, Georgia, and Crystal Lake, Illinois, alongside human factors studies on button layouts to optimize user interaction.[6] These innovations by key figures such as Bell engineers focused on enhancing dialing speed and reliability, paving the way for commercial push-button systems in the 1960s.Commercial Introduction
The Bell System conducted the first field trial of dual-tone multi-frequency (DTMF) push-button telephone service on November 1, 1960, in Findlay, Ohio, marking the initial public availability of this technology following earlier prototypes from the 1940s and 1950s.[8] This rollout involved equipping local exchanges with modified #5 Crossbar switches to support the new signaling method, allowing a limited number of subscribers to use push-button phones for local calls.[8] On November 18, 1963, the Bell System officially introduced touch-tone service, starting in the communities of Carnegie and Greensburg, Pennsylvania.[3][1] The service featured the Western Electric Model 1500 telephone, the first production push-button desk set with a 10-button keypad arranged in a 3x3 grid plus a zero, designed to replace rotary dials while remaining backward-compatible with existing pulse systems.[1][9] Customers paid an extra monthly fee of $1.50 for the service, reflecting its premium status at launch.[10] In the late 1960s, the Western Electric 2500 series emerged as an early successor, introducing integrated push-button keypads in a more compact design compatible with evolving network infrastructure.[9] However, initial adoption faced challenges, including higher equipment and service costs compared to rotary phones, as well as limited compatibility requiring central office upgrades to handle DTMF signaling without disrupting pulse dialing.[10][11] By 1968, the standard keypad was enhanced with the addition of the asterisk (*) and pound (#) keys, expanding to a 12-button layout in models like the 2500 series to support advanced signaling for future applications such as data transmission.[9][12]Adoption and Legacy
The adoption of push-button telephones accelerated in the 1970s, driven by advancements in metal–oxide–semiconductor (MOS) integrated circuit technology that enabled cheaper production and the integration of digital features in telephone designs. British companies such as Pye TMC, Marconi-Elliott, and GEC pioneered digital push-button telephones using MOS technology as early as 1970, which facilitated more compact and cost-effective manufacturing processes compared to earlier analog systems. This technological shift contributed to broader market penetration by reducing costs and allowing for enhanced functionalities like electronic tone generation.[13] Globally, adoption varied by region, with earlier implementations in parts of Europe and Asia. In the Netherlands, Philips developed a direct current (DC) signaling push-button dialing system in the 1950s for use in models like the UB-49 telephone, marking one of the continent's initial forays into button-based interfaces ahead of widespread dual-tone multi-frequency (DTMF) standards. Japan saw commercial introduction of push-button telephones in 1969 through Nippon Telegraph and Telephone (NTT) and manufacturers like OKI, aligning with the country's economic boom and NTT's vision for faster dialing systems; by the early 1970s, production shifted fully from rotary models to electronic push-button variants. In contrast, broader European rollout, such as Sweden's adoption of Ericsson's Diavox touch-tone phones in 1978 as the first mass-produced model, followed more gradually due to national telecom monopolies.[7][14][15] From ETHW: By 1976, Bell had equipped 70 percent of its exchanges for touch-tone service.[16] By the late 1970s and early 1980s, touch-tone dialing had largely replaced rotary systems in the United States, with push-button phones becoming the standard in households as infrastructure support expanded.[4] Full dominance was achieved by the 1990s, as rotary pulse dialing was phased out in favor of DTMF-compatible systems across telecom networks.[17] The legacy of push-button telephones extends as a bridge to digital telephony, with their 12-key layout directly influencing the numeric keypads on mobile phones and other devices since the 1980s. DTMF signaling from push-button systems remains integral to Voice over IP (VoIP) standards, enabling interactive voice response and call routing in modern networks. Post-1990s, push-button phones persist in landline systems, supported by analog telephone adapters (ATAs) that ensure compatibility with VoIP services, allowing legacy devices to function alongside smartphones and digital infrastructure.[18][19]Signaling Methods
Dual-Tone Multi-Frequency (DTMF)
Dual-tone multi-frequency (DTMF) is a telecommunication signaling protocol employed in push-button telephones to transmit dialed digits and symbols using pairs of simultaneous audio tones within the voice-frequency band. Each signal consists of one tone from a low-frequency group—697 Hz, 770 Hz, 852 Hz, and 941 Hz—and one from a high-frequency group—1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz—ensuring unique identification without overlap in the telephone network's audio spectrum.[20] These frequencies conform to the international standard outlined in ITU-T Recommendation Q.23, which specifies their allocation for reliable detection across analog subscriber lines and network paths.[21] The specific frequency pairs correspond to the keys on the standard telephone keypad as follows:| Key | Low Frequency (Hz) | High Frequency (Hz) |
|---|---|---|
| 1 | 697 | 1209 |
| 2 | 697 | 1336 |
| 3 | 697 | 1477 |
| 4 | 770 | 1209 |
| 5 | 770 | 1336 |
| 6 | 770 | 1477 |
| 7 | 852 | 1209 |
| 8 | 852 | 1336 |
| 9 | 852 | 1477 |
| * | 941 | 1209 |
| 0 | 941 | 1336 |
| # | 941 | 1477 |
Pulse Dialing
Pulse dialing in push-button telephones adapted the rotary dial's loop-interruption technique for compatibility with older electromechanical exchanges, allowing electronic keypads to generate timed breaks in the direct current (DC) loop current. Each digit is represented by a specific number of pulses, where the digit "5," for instance, produces five interruptions, typically at a rate of 10 pulses per second with each pulse consisting of a 60-millisecond break (open circuit) and a 40-millisecond make (closed circuit).[26][27] In push-button implementations, electronic timing circuits simulate these rotary breaks when a key is pressed and held, using components like transistors or integrated circuits controlled by a microcomputer to open and close the loop the required number of times for each digit.[28] Early models often included a user-selectable switch to toggle between pulse and dual-tone multi-frequency (DTMF) modes, enabling operation on legacy step-by-step or panel exchanges that did not yet support tones; this feature was prevalent in transitional telephones from the 1970s and 1980s, such as certain Bell System models.[1] Compared to DTMF, which uses simultaneous audio tones for faster signaling, pulse dialing is slower, potentially taking up to 10 seconds to transmit a full 10-digit number due to the sequential interruptions and inter-digit pauses of at least 600 milliseconds.[29] It is also more prone to errors from electrical noise or line interference, which can distort the pulse count and lead to incorrect digit registration.[30] Pulse dialing's use in push-button phones declined with the widespread upgrade to electronic switches supporting DTMF by the late 20th century, though it persisted in some rural and legacy analog systems into the 2000s for backward compatibility.[31]DC Signaling
DC signaling in push-button telephone systems relies on variations in direct current (DC) flow through the telephone line loop to convey operational states and dialing information, primarily for on-premises and early private branch exchange (PBX) applications. When a telephone goes off-hook, it closes the loop, allowing a typical current of 20-50 mA to flow from the central office or PBX battery supply, signaling the system to provide dial tone and establish a connection.[32] This current modulation, achieved through resistance changes or relay operations, enables the detection of dialing digits without relying on audio frequencies, making it suitable for local intra-office calling where noise immunity is essential.[33] In the 1950s, Philips pioneered a DC signaling method specifically for push-button dialing in PBX environments, such as their UB-49 system, where buttons trigger relay-based changes in loop current or voltage drops to encode digits. This approach involved switching resistors in the telephone set to alter the loop resistance—typically values like 1000 Ω, 2700 Ω, or 7500 Ω—causing detectable decreases in line current that the exchange interprets as specific numbers.[34] The nominal line voltage was 48 V DC, with resistance-based detection at the PBX allowing reliable operation over short distances without the need for tone generation.[35] Polarity reversal could further distinguish signals, enhancing error detection for simultaneous key presses. This DC method integrated seamlessly into push-button phones for industrial and office settings, where buttons activated relays to pulse or level-shift the current for digit transmission, offering a quieter alternative to mechanical pulse dialing for local calls. It remained compatible with pulse methods for external lines but found primary use in closed PBX networks. By the post-1980s era, the shift to digital PBX systems largely relegated DC signaling to niche industrial applications, as electronic switching favored more scalable digital protocols.[36]Keypad Design
Standard Layout
The standard layout of a push-button telephone keypad consists of a 3-by-4 rectangular grid comprising 12 buttons, arranged with the digits 1 through 9 in three rows of three (1-2-3 at the top, 4-5-6 in the middle, and 7-8-9 at the bottom), followed by a bottom row featuring an asterisk (*) to the left of the zero (0) and a pound or hash symbol (#) to the right.[37] This configuration was developed by researchers at Bell Laboratories in the late 1950s through human factors testing led by psychologist Alphonse Chapanis, who evaluated multiple arrangements for speed and accuracy in dialing, ultimately selecting the vertical columnar progression of digits to mimic the natural reading order while optimizing ergonomic reach on the handset.[38] Each digit button from 2 through 9 incorporates alphabetical groupings inherited from rotary dial telephones, such as ABC on 2, DEF on 3, GHI on 4, JKL on 5, MNO on 6, PQRS on 7, TUV on 8, and WXYZ on 9, with 1 and 0 typically lacking letters (though in early designs Q and Z were omitted or added to 1, the standard places them on 7 and 9); this design facilitated mnemonic recall of phone numbers from printed directories, where exchanges like "KL5" corresponded to 555, easing user familiarity during the transition from rotary systems.[38][39][37] The layout has been mandated for interoperability in telecommunications equipment by international and national standards since the late 1960s, including ITU-T Recommendation E.161 (originally issued in 1988 but building on earlier Bell System specifications from 1963) for the arrangement of digits, letters, and symbols on telephones, and ANSI/TIA/EIA-470 (first published in 1987 with roots in 1968 compatibility requirements for touch-tone service) for telephone terminal performance, ensuring uniform dialing across networks.[37][40] Physically, the buttons are constructed from durable plastic with raised edges or beveled surfaces to provide tactile feedback and prevent accidental presses, operating via mechanical switches (such as snap-action domes or conductive rubber contacts) beneath each key that complete an electrical circuit upon depression, though some modern variants employ capacitive sensing for touch detection without moving parts.[38][41] For accessibility, adaptations such as removable Braille overlays—thin plastic or silicone sheets with raised dots corresponding to digits and letters—can be applied to the keypad, enabling visually impaired users to independently dial by touch, as offered in products compliant with guidelines like those from the American Foundation for the Blind.[42][43]| Row | Left | Middle | Right |
|---|---|---|---|
| 1 | 1 | 2 (ABC) | 3 (DEF) |
| 2 | 4 (GHI) | 5 (JKL) | 6 (MNO) |
| 3 | 7 (PQRS) | 8 (TUV) | 9 (WXYZ) |
| 4 | * | 0 | # |