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 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.
Early telephone exchanges signaled the switchboard operator when a subscriber picked up the telephone handset to make a call. The operator answered requesting the destination of the call. When manual exchanges were replaced with automated switching systems, the exchange generated a tone played to the caller when the telephone set was placed off-hook, indicating that the system was live and a telephone number could be dialed. Each digit was transmitted as it was dialed which caused the switching system to select the desired destination circuit. Modern electronic telephones may store the digits as they are entered, and only switch off-hook to complete the dialing when the subscriber presses a call or talk button.
Invented by engineer August Kruckow, the dial tone was first used in 1908 in Hildesheim, Germany. In the United States, the 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 show Eisenhower how to use a rotary dial phone.
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 was a 600 Hz tone that was amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current. In the UK, the standard Post Office dialling tone was 33 Hz; it was generated by a motor-driven ringing machine in most exchanges, and by a vibrating-reed generator in the smaller ones. Some later ringing machines also generated a 50 Hz dial tone.
The modern dial tone varies between countries. The Precise Tone Plan for the North American Numbering Plan of the US, Canada, and Mexico specifies a combination of two tones (350 Hz and 440 Hz) which when mixed appear to have a modulation at 90 Hz. These two tones are also used as a dial tone in the UK but most of Europe uses a constant single tone of 425 Hz.
Cellular telephone services do not generate dial tones as no connection is made until the entire number has been specified and transmitted.
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 so as to remind users to dial a prefix for, or select in another way, an outside line.
A secondary dial tone is a dial tone-like sound presented to the caller after a call has been set up to prompt for additional digits to be dialed. Secondary dial tones are often used in call queuing and call forwarding systems.
A stutter dial tone is a rapidly interrupted tone used to indicate special service conditions. It may serve as a message-waiting indicator for voice mail, or indicate that a calling feature, such as call forwarding has been activated.
A soft dial tone or express dial tone may be used when no actual service active on a line, and normal calls cannot be made. 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. 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 technicians in order to facilitate installation or activation.