Dial tone

dial-tonedialling tonestutter dial tonetone
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.wikipedia
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Telephone exchange

exchangescentral officeexchange
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.
Central office locations may also be identified in North America as wire centers, designating a facility from which a telephone obtains dial tone.

Telephone call

callphone callphone calls
It indicates that the exchange is working and is ready to initiate a telephone call.
Otherwise, the calling party is usually given a tone to indicate they should begin dialing the desired number.

Special information tone

special information tonesOOS
If no digits are forthcoming, the permanent signal 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.
Like a dial tone or busy signal, the SIT is an in-band signal intended both to be heard by the caller, and to be detected by automated dialing equipment to determine a call has failed.

Off-hook tone

high toneHowler/off-hook warningoff-hook
If no digits are forthcoming, the permanent signal 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.
It is played between the dial tone timeout recording ("If you would like to make a call...") and the permanent signal holding state.

Call forwarding

forwardingCall divertCall Forward on Busy
It may serve as a message-waiting indicator for voice mail, or indicate that a calling feature, such as call forwarding has been activated.
More consistently, the forwarded line indicates its condition by stutter dial tone.

Precise Tone Plan

The Precise Tone Plan for the North American Numbering Plan of the US, Canada, and various Caribbean nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, appear to have a modulation at 90 Hz. The UK dial tone is extremely similar, but combines two 350 Hz and 450 Hz tones instead.
Dial tone is a continuous tone of a combination of the frequencies 350 and 440 Hz at a level of −13 dBm

Message-waiting indicator

Message Waiting IndicationMessage waiting indicationsmessage waiting indicator
It may serve as a message-waiting indicator for voice mail, or indicate that a calling feature, such as call forwarding has been activated.
VMWI differs from existing features that use other message indicators, such as audible stuttered dial tone, in that it activates a visual indicator on the CPE.

Rotary dial

dialrotary telephonerotary phone
When he picked up his own household phone his assistant had to explain what the strange noise was, as well as show Eisenhower how to use a rotary dial phone.
Dial tone

9-1-1

911911 call9-1-1 call
It is maintained only so that an attached phone can dial the emergency telephone number (such as 911, 112 or 999), in compliance with the law in most places.
On wired (land line) phones, this usually is accomplished by a "soft" dial tone, which sounds normal but will allow only emergency calls (or, in some cases, will state the number of the local telephone company which may be called in order to initiate regular service).

Busy signal

engaged tonebusybusy tone
Busy signal
In some phone companies in the United Kingdom, the busy signal is played after the dial tone to indicate the caller has used up their allocated time to dial a number and must hang up, before the off-hook tone is played.

Ringing tone

ringback toneringback tonesAudible ringing
Ringback tone
Dial tone

Telephony

digital telephonytelephonedigital
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.

Signaling (telecommunications)

signalsignalingsignalling
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.

On- and Off-hook

off-hookon-hookon hook
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.

Permanent signal

left off-hookpermanent loop
If no digits are forthcoming, the permanent signal 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.

Intercept message

recorded announcementrecordingerror messages
If no digits are forthcoming, the permanent signal 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.

Switchboard operator

telephone operatoroperatoroperators
Early telephone exchanges signaled the switchboard operator when a subscriber picked up the telephone handset to make a call.

Hildesheim

HIHildesheim, DEHildesheim, Germany
Invented by engineer August Kruckow, the dial tone was first used in 1908 in Hildesheim, Germany.

Dwight D. Eisenhower

EisenhowerDwight EisenhowerPresident Eisenhower
When President Dwight D. Eisenhower retired in 1961 it was nearly universal, but the president himself had never been confronted with a dial tone.

Electromechanics

electromechanicalelectro-mechanicalelectromechanical engineering
Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means.

Hertz

MHzkHzHz
In the United States, the standard "city" dial tone was a 600 Hz tone that was amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current.

North American Numbering Plan

Area codeArea codesArea code(s)
The Precise Tone Plan for the North American Numbering Plan of the US, Canada, and various Caribbean nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, appear to have a modulation at 90 Hz. The UK dial tone is extremely similar, but combines two 350 Hz and 450 Hz tones instead.

United States

American🇺🇸U.S.
The Precise Tone Plan for the North American Numbering Plan of the US, Canada, and various Caribbean nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, appear to have a modulation at 90 Hz. The UK dial tone is extremely similar, but combines two 350 Hz and 450 Hz tones instead.

Canada

🇨🇦CanadianCAN
The Precise Tone Plan for the North American Numbering Plan of the US, Canada, and various Caribbean nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, appear to have a modulation at 90 Hz. The UK dial tone is extremely similar, but combines two 350 Hz and 450 Hz tones instead.

Caribbean

West Indiesthe CaribbeanWest Indian
The Precise Tone Plan for the North American Numbering Plan of the US, Canada, and various Caribbean nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, appear to have a modulation at 90 Hz. The UK dial tone is extremely similar, but combines two 350 Hz and 450 Hz tones instead.