Dialling (telephony)

[1][2] Under Strowger's system, which was first introduced commercially in La Porte, Indiana, the number was dialled using two telegraph keys.

[2] From 1912, the British General Post Office, which also operated the British telephone system, installed several automatic telephone exchanges from several vendors in trials at Darlington on 10 October 1914 (rotary system), Fleetwood (relay exchange from Sweden), Grimsby (Siemens), Hereford (Lorimer) and Leeds (Strowger).

The Director systems used SXS switches for destination routing and number translation facilities similar to the register used in common-control exchanges.

Each column and row has a distinct frequency assigned, thus generating a total of sixteen dual-tone multi-frequency (DTMF) signals.

Initial pushbutton designs employed mechanical switches, so that each button activated certain combinations of capacitors and inductors of an oscillator, while later versions used semiconductor logic chips to synthesize the frequencies.