A telephone keypad is a keypad that appears on a "Touch Tone" telephone. It was standardised when the dual-tone multi-frequency system in the new push-button telephone was introduced in the 1960s, which gradually replaced the rotary dial. The invention of the keypad is attributed to John E. Karlin, an industrial psychologist at Bell Labs. The contemporary keypad is laid out in a 4×3 grid, although the original DTMF system in the new keypad had an additional column for four now-defunct menu selector keys.
Key Frequency 
When used to dial a telephone number, pressing a single key will produce a pitch consisting of two simultaneous pure tone sinusoidal frequencies. The row in which the key appears determines the low frequency, and the column determines the high frequency. For example, pressing the '1' key will result in a sound composed of both a 697 and a 1209 hertz (Hz) tone.
|1209 Hz||1336 Hz||1477 Hz|
The "*" is called the "star key" or "asterisk key". "#" is called the "number sign", "pound key", "hash key", "hashtag", hex key, "octothorpe" or "square", depending on one's nationality or personal preference. These can be used for special functions. For example, in the UK, users can order a 7.30am alarm call from a British Telecom telephone exchange by dialling: *55*0730#.
Most of the keys also bear letters according to the following system:
- 0 = none (in some telephones, "OPERATOR" or "OPER")
- 1 = none (in some older telephones, QZ)
- 2 = ABC
- 3 = DEF
- 4 = GHI
- 5 = JKL
- 6 = MNO
- 7 = PQRS
- 8 = TUV
- 9 = WXYZ
These letters have had several auxiliary uses. Originally, they referred to exchanges. In the mid-20th century United States, before the advent of All-Number Calling, numbers were seven digits long including a two-digit prefix which was expressed as the letters rather than numbers e.g.; KL5-5445. The UK telephone numbering system used a similar two-letter code after the initial zero to form the first part of the subscriber trunk dialling code for that region – for example, Aylesbury was assigned 0AY6 which translated into 0296. (The majority of these original numbers have remained, particularly in the rural areas, and are currently still in service. The modern equivalent of 0AY6, namely 01296, still refers to Aylesbury.)
The letters have also been used, mainly in the United States, as a way of remembering telephone numbers easily. For example, an interior decorator might license the phone number 1-800-724-6837 but advertise it as the more memorable phoneword 1-800-PAINTER. Sometimes businesses advertise a number with a mnemonic word having more letters than there are digits in the phone number. Usually, this means that you just stop dialing at 7 digits after the area code or that the numbers are ignored by the switchboard.
In recent times, the letters on the keys are needed also for entering text on mobile phones, for text messaging, entering names in the phone book, etc.; multi-tap and predictive text systems are used.
Letter mapping 
When designing or selecting a new phone, publishing or using phonewords, one should be aware that there have been multiple standards for the mapping of letters (characters) to numbers (keypad layouts, as with keyboard layout) on telephone keypads over the years.
The system used in Denmark was different from that used in the U.K., which was different from the U.S. etc. The use of alphanumeric codes for exchanges was abandoned in Europe when international direct dialling was introduced in the 1960s, because, for example, dialling VIC 8900 on a Danish telephone would result in a different number than dialling it on a British telephone. At the same time letters were no longer put on the dials of new telephones.
The keypad pictured above is mapped according to the current international standard. The ITU established an international standard (ITU E.161) in the mid-1990s, and that should be the layout used for any new devices.
There is a standard that covers European languages and other languages used in Europe, published by independent ETSI organisation: ETSI ES 202 130; first published in 2003 and updated in 2007. (Work describing some principles of the standard is available  .)
Since many newer smartphones (such as PalmPilot and BlackBerry) have full keyboards instead of the traditional telephone keypads, the user must execute additional steps to dial a number containing convenience letters. On certain BlackBerry devices, a user can press the Alt key, followed by the desired letter, and the device will generate the appropriate DTMF tone.
The first telephony application that did not deploy a dial pad is Blink (a screenshot of the pad-less audio interface is available here). As computers benefit of a full keyboard, the developers felt that it was more natural to allow typing DTMF tones by using the computer keyboard while in a middle of an audio session without having to present the end-user an explicit dial-pad graphical user interface.
Keypads may be illuminated from inside, especially on equipment for mobile use. This allows a certain sector of the telephone keypad to be visible in darkness.
See also 
- Engineering Pathway – Bell Telephone introduces push button telephone – by Alice Agogino – November 18th, 2009
- "Monmouth man, inventor of touch-tone keypad, dies at 94". The Star-Ledger. February 9, 2013. Retrieved 2013-02-09.
- Phone Key Pads
- E.161 : Arrangement of digits, letters and symbols on telephones and other devices that can be used for gaining access to a telephone network
- ETSI (2003-10-29), ETSI ES 202 130 Ver. 1.1.1: Human Factors (HF); User Interfaces; Character repertoires, ordering rules and assignments to the 12-key telephone keypad, ETSI, retrieved 2011-11-03
- ETSI (2007-09-06), ETSI ES 202 130 Ver. 2.1.2: Human Factors (HF); User Interfaces; Character repertoires, orderings and assignments to the 12-key telephone keypad (for European languages and other languages used in Europe), ETSI
- Böcker, Martin; Bruno von Niman, Karl Ivar Larsson (2006-09-01), "Increasing text-entry usability in mobile devices for languages used in Europe", Interactions 13 (5): 30, doi:10.1145/1151314.1151336, ISSN 10725520, CiteSeerX: 10.1.1.125.7511
- Blackberry Tips, PC World, October 2005.