A dial tone (dialling tone in the UK) 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 partial dial 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 to the caller when the telephone set was picked up, indicating that the system was ready to accept dialed digits. 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 button.

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

The Bell Telephone Manufacturing Company (BTMC) in Antwerp, Belgium, Western Electric's international subsidiary, first introduced dial tone as a standard facility with the cutover of the 7A Rotary Automatic Machine Switching System at Darlington, England, on 10 October 1914. Dial tone was an essential feature, because the 7A Rotary system was a common control switching system. It used the dial tone to indicate to the user that the switching system was ready to accept digits.

In the United States, dial tone was introduced in the 1920s. By the time President Dwight D. Eisenhower retired in 1961 it was nearly universal, but the president himself had never encountered 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 devices such as motor–generators or vibrators. 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 dialing 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 various Caribbean nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, creates a beat frequency of 90 Hz. The UK dial tone is similar, but combines 350 Hz and 450 Hz tones instead, creating a 100 Hz beat frequency. Most of Europe, as well as much of Latin America and Africa, uses a constant single tone of 425 Hz. France currently uses a single 440 Hz tone and Japan uses a single 400 Hz tone.

Cellular telephone services do not generate dial tones as no connection is made until the entire number has been specified and transmitted.^{[citation needed]}