Dial Tone - History

History

In the early days of telephony a telephone operator 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 entering a number without the need to first go off-hook. A modern telephone stores the digits as they are entered, 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 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 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 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 generated dialling tone at 50 Hz.

The modern dial tone varies between countries, being a "buzz" of two interfering tones (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. Modern UK dialling tone is also 350 plus 440 Hz. Modems, fax machines, and auto dialers must be designed to recognise these so-called call-progress tones, as well as comply with differing standards and regulations.

Dial tones are not relevant to GSM cellular phones, and GSM 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.

The US dial tone's two frequencies correspond to the standard concert pitch of A440, and approximately the "F".