Pulse dialing

The pulse repetition rate was historically determined based on the response time needed for electromechanical switching systems to operate reliably.

[1] The first commercial automatic telephone exchange, designed by Almon Brown Strowger, opened in La Porte, Indiana on 3 November 1892, and used two telegraph-type keys on the telephone, which had to be operated the correct number of times to control the vertical and horizontal relay magnets in the exchange.

The pulses were sent as the user rotated the dial to the finger stop starting at a different position for each digit transmitted.

Operating the dial error-free required smooth rotary motion of the finger wheel by the user, but was found as too unreliable.

In the United Kingdom, it was once possible to make calls from coin-box phones by tapping the switch hook without depositing coins.

In the 1940s, Bell Laboratories conducted field trials of pushbutton telephones for customer dialing to determine accuracy and efficiency.

In 1963, the Bell System introduced to the public dual-tone multi-frequency (DTMF) technology under the name Touch-Tone, which was a trademark in the U.S. until 1984.

Some keypad telephones have a switch or configuration method for the selection of tone or pulse dialing.

Mobile telephones and most voice-over-IP systems use out-of-band signaling and do not send any digits until the entire number has been keyed by the user.

Many VoIP systems are based on the Session Initiation Protocol (SIP), which uses a form of Uniform Resource Identifiers (URI) for addressing, instead of digits alone.