Dial tone

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.

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