Strowger (courtesy invent.org)
Almon Strowger was an undertaker with a problem in the late 1800s: His customers' phone calls often weren't reaching him, because the human telephone operators in charge of connecting their calls often misdirected them. The operators generally worked with a huge switchboard and had to manually make the required connection from caller to receiver.
Frustrated with the constant switching errors, Strowger developed a device consisting of buttons that a caller could tap to signal a central switch; at the central switch, a rotating mechanical arm would move the caller's line until it made contact with the number that had been tapped in. The automatic telephone exchange was born.
The beauty of the automatic telephone exchange was its simplicity and scalability. How well did it scale? Strowger's device, with few modifications, remained the backbone of telecommunications from the time the first automatic telephone exchange was installed in 1892, until the late 1970s, when touch-tone dialing became the standard.