Dial tone
Encyclopedia
A dial tone is a telephony
signal used to indicate that the telephone exchange
is working, has recognized an off-hook
, and is ready to accept a call. The tone stops when the first numeral is dialed. If no digits are forthcoming, the permanent signal
procedure is invoked, often eliciting a special information tone
.
would answer when a subscriber picked up the telephone to make a call. When operators were replaced by automated systems an automatically generated tone was substituted to signify that the system was live and a call could be dialed. It was required to pick up the receiver (go off-hook) before dialing, and each digit was transmitted as it was dialed. This is unlike modern telephones, which also allow dialing a number without the need to first go off-hook. The telephone stores the digits as they are
dialed, and only goes off-hook and actually sends the digits when the subscriber presses a "Call" or "Talk" button.
In the United States, dial tone was introduced in the 1940s, and became widespread in the 1950s. When President Dwight D. Eisenhower
retired in 1961 it was nearly universal, but the president himself had not been confronted with a dial tone. When he picked up his own household phone his assistant had to explain what the strange noise was, as well as how to use a rotary phone dial.
Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means; in the United States, the standard "city" dial tone consisted of a 600 Hz
tone amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current. In the UK, the standard Post Office dial tone was 330 Hz: this was generated by a motor-driven ringing machine in most exchanges, and by a vibrating-reed generator in the smaller ones.
The modern dial tone varies between countries, being a "buzz" of two interfering
tone
s (350 Hz and 440 Hz, as defined in the Precise Tone Plan
) in the North American Numbering Plan
(most of North America
), and a constant single tone (425 Hz) in most of Europe
. Modem
s, fax machines, and autodialers must be designed to recognise these so-called call-progress tone
s, as well as comply with differing standards and regulations.
Dial tones are not relevant to mobile phone
s, and mobile phone subscribers will never hear a dial-tone. Subscribers place a call much like they would with a modern telephone, by dialing the digits and pressing a "Call" or "Talk" button. If the mobile subscriber is connected to a mobile network, then the call will be placed.
Experienced musicians will be able to tune their stringed instruments to a US dial tone, since its two frequencies correspond to the standard concert pitch
of A440, and (approximately) the "F
".
Private or internal PBX or key phone systems also have their own dial tone, sometimes the same as the external PSTN one, and sometimes different to remind users to dial a prefix for—or select in another way—an outside telephone line
.
Secondary dial tone
A secondary dial tone, or second dial tone, is a dial tone-like sound presented to the caller after a call has already been set up. Secondary dial tones are often used in call queuing and call forwarding systems.
Unlike a normal dial tone, a secondary dial tone is provided when a connection has already been established and, except for free calls, is being charged for. Systems using secondary dialtone have been criticized for misleading callers into thinking that they are not yet being charged.
Stutter dial tone
A "stuttered" or interrupted dial tone is often used to indicate that voice mail is waiting (see Message Waiting Indicator
), or that a calling feature such as call forwarding
has been activated.
(such as 9-1-1
, 1-1-2
or 9-9-9
), in compliance with the law in most places. It can sometimes call the business office of the local exchange carrier
which owns or last leased the line, such as via 6-1-1
. Other functions such as ringback
or ANAC
may also be accessed by technician
s in order to facilitate installation or activation.
Often, a new telephone number is assigned to the line so that it can function, but callback is restricted, and end-user
s do not know the number. These numbers may be outside the normal range used for regular lines, potentially causing trouble when telephone numbering plan
s are changed.
Deactivated lines can also be maintained with no dial tone at all, while still connected to and powered by the switch, in a state sometimes called INB or Installation Busy.
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
signal used to indicate that the telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
is working, has recognized an off-hook
Off-hook
In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...
, and is ready to accept a call. The tone stops when the first numeral is dialed. If no digits are forthcoming, the permanent signal
Permanent signal
Permanent Signal in US telephony jargon, or Permanent Loop in British usage, is a condition in which a POTS line is off-hook without connection for an extended period of time. This is indicated in modern switches by the silent termination after the off hook tone times out and the telephone...
procedure is invoked, often eliciting a special information tone
Special information tone
In telephony, a special information tone is an international standard three beep signal indicating a call did not go through and usually precedes a recorded announcement explaining the problem...
.
History
In the early days of telephony a telephone operatorTelephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...
would answer when a subscriber picked up the telephone to make a call. When operators were replaced by automated systems an automatically generated tone was substituted to signify that the system was live and a call could be dialed. It was required to pick up the receiver (go off-hook) before dialing, and each digit was transmitted as it was dialed. This is unlike modern telephones, which also allow dialing a number without the need to first go off-hook. The telephone stores the digits as they are
dialed, and only goes off-hook and actually sends the digits when the subscriber presses a "Call" or "Talk" button.
In the United States, dial tone was introduced in the 1940s, and became widespread in the 1950s. When President Dwight D. Eisenhower
Dwight D. Eisenhower
Dwight David "Ike" Eisenhower was the 34th President of the United States, from 1953 until 1961. He was a five-star general in the United States Army...
retired in 1961 it was nearly universal, but the president himself had not been confronted with a dial tone. When he picked up his own household phone his assistant had to explain what the strange noise was, as well as how to use a rotary phone dial.
Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means; in the United States, the standard "city" dial tone consisted of a 600 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
tone amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current. In the UK, the standard Post Office dial tone was 330 Hz: this was generated by a motor-driven ringing machine in most exchanges, and by a vibrating-reed generator in the smaller ones.
The modern dial tone varies between countries, being a "buzz" of two interfering
Beat (acoustics)
In acoustics, a beat is an interference between two sounds of slightly different frequencies, perceived as periodic variations in volume whose rate is the difference between the two frequencies....
tone
Pitch (music)
Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...
s (350 Hz and 440 Hz, as defined in the Precise Tone Plan
Precise Tone Plan
The Precise Tone Plan specification for the North American network defines the Call-progress tones used for standard signaling in the PSTN. The tones are as follows:* dial tone is a continuous tone having frequencies of 350 and 440 Hz at a level of −13 dBm...
) in the North American Numbering Plan
North American Numbering Plan
The North American Numbering Plan is an integrated telephone numbering plan administered by Neustar which encompasses 24 countries and territories, including the United States and its territories, Canada, Bermuda, and 16 nations of the Caribbean...
(most of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
), and a constant single tone (425 Hz) in most of Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. Modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s, fax machines, and autodialers must be designed to recognise these so-called call-progress tone
Call-progress tone
In telephony, call progress tones are audible tones sent from the public switched telephone network central office or a private branch exchange to calling parties to indicate the status of phone calls....
s, as well as comply with differing standards and regulations.
Dial tones are not relevant to mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s, and mobile phone subscribers will never hear a dial-tone. Subscribers place a call much like they would with a modern telephone, by dialing the digits and pressing a "Call" or "Talk" button. If the mobile subscriber is connected to a mobile network, then the call will be placed.
Experienced musicians will be able to tune their stringed instruments to a US dial tone, since its two frequencies correspond to the standard concert pitch
Concert pitch
Concert pitch refers to the pitch reference to which a group of musical instruments are tuned for a performance. Concert pitch may vary from ensemble to ensemble, and has varied widely over musical history...
of A440, and (approximately) the "F
F (musical note)
F is a musical note, the fourth above C. It is also known as fa in fixed-do solfège.When calculated in equal temperament with a reference of A above middle C as 440 Hz, the frequency of Middle F is approximately 349.228 Hz. See pitch for a discussion of historical variations in...
".
Variants
Second dial tonePrivate or internal PBX or key phone systems also have their own dial tone, sometimes the same as the external PSTN one, and sometimes different to remind users to dial a prefix for—or select in another way—an outside telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
.
Secondary dial tone
A secondary dial tone, or second dial tone, is a dial tone-like sound presented to the caller after a call has already been set up. Secondary dial tones are often used in call queuing and call forwarding systems.
Unlike a normal dial tone, a secondary dial tone is provided when a connection has already been established and, except for free calls, is being charged for. Systems using secondary dialtone have been criticized for misleading callers into thinking that they are not yet being charged.
Stutter dial tone
A "stuttered" or interrupted dial tone is often used to indicate that voice mail is waiting (see Message Waiting Indicator
Message Waiting Indicator
A message-waiting indicator in telephony, is a Telcordia Technologies term for an FSK-based telephone calling feature that illuminates an LED on select telephones to notify a telephone user of waiting voicemail messages on most North American public telephone networks and PBXs.As described in...
), or that a calling feature such as call forwarding
Call forwarding
Call forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...
has been activated.
"Soft" dial tones
A "soft" dial tone, less often called "secondary" dial tone or "express" dial tone, is audibly the same as a regular one, except that there is no actual service active on the line, and normal calls cannot be made. It is maintained only so that an attached phone can dial the emergency telephone numberEmergency telephone number
Many countries' public telephone networks have a single emergency telephone number, sometimes known as the universal emergency telephone number or occasionally the emergency services number, that allows a caller to contact local emergency services for assistance. The emergency telephone number may...
(such as 9-1-1
9-1-1
9-1-1 is the emergency telephone number for the North American Numbering Plan .It is one of eight N11 codes.The use of this number is for emergency circumstances only, and to use it for any other purpose can be a crime.-History:In the earliest days of telephone technology, prior to the...
, 1-1-2
1-1-2
112 is the principal emergency telephone number that can be dialed free of charge from any telephone or any mobile phone in order to reach emergency services in the European Union , its candidates for accession, members of the EEA agreement, as well as several other countries in the world...
or 9-9-9
999 (emergency telephone number)
999 is an official emergency telephone number in a number of countries which allows the caller to contact emergency services for urgent assistance....
), in compliance with the law in most places. It can sometimes call the business office of the local exchange carrier
Local exchange carrier
Local Exchange Carrier is a regulatory term in telecommunications for the local telephone company.In the United States, wireline telephone companies are divided into two large categories: long distance and local...
which owns or last leased the line, such as via 6-1-1
6-1-1
For customers of some telephone companies in Canada and in the U.S., 611 is the abbreviated dialing telephone number used to report a problem with telephone service, or with a payphone...
. Other functions such as ringback
Ringback number
Ringback number is a number used by many telephone company linemen/technicians to test whether or not a new telephone line installation is working correctly and the phone number is routing properly....
or ANAC
Automatic number announcement circuit
An automatic number announcement circuit is a special telephone number that is meant to be used by phone company technicians and other telecommunications technicians to determine the phone number of a particular line....
may also be accessed by technician
Technician
A technician is a worker in a field of technology who is proficient in the relevant skills and techniques, with a relatively practical understanding of the theoretical principles. Experienced technicians in a specific tool domain typically have intermediate understanding of theory and expert...
s in order to facilitate installation or activation.
Often, a new telephone number is assigned to the line so that it can function, but callback is restricted, and end-user
End-user
Economics and commerce define an end user as the person who uses a product. The end user or consumer may differ from the person who purchases the product...
s do not know the number. These numbers may be outside the normal range used for regular lines, potentially causing trouble when telephone numbering plan
Telephone numbering plan
A telephone numbering plan is a type of numbering scheme used in telecommunications to allocate telephone numbers to subscribers and to route telephone calls in a telephone network. A closed numbering plan, such as found in North America, imposes a fixed total length to numbers...
s are changed.
Deactivated lines can also be maintained with no dial tone at all, while still connected to and powered by the switch, in a state sometimes called INB or Installation Busy.
External links
- World PSTN Tone Database - Dial and other telephone tones which can be listened to (free registration required)