First announced in 1955, the first customer trial installation of an all-electronic central office commenced in Morris, Illinois in November 1960 by Bell Laboratories.
Just three years later, in September 1968, Britain's Post Office opened the world's first all-digital pulse-code modulation (PCM) exchange named Empress (three decades after British scientist Alec Reeves had invented the PCM encoding system without the digital components to take full advantage).
[2] Other nations vying to reach the forefront of technical innovation would adopt metal–oxide–semiconductor (MOS) and PCM technologies to make their own transitions from analog to digital telephony throughout the '70s.
Time-division multiplexing (TDM) technology permitted the simultaneous transmission of multiple telephone calls on a single wire connection between central offices or other electronic switches, resulting in dramatic capacity improvements of the telephone network.
With the advances of digital electronics starting in the 1960s telephone switches employed semiconductor device components in increasing measure.