The wiring plans evolved into modular hardware building blocks with a variety of functionality and services in the 1A key telephone system developed in the Bell System in the 1930s.
The General Post Office (GPO) of the United Kingdom carried also the sole responsibility for providing telecommunication services across the country.
With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps.
Key telephone systems are primarily defined by arrangements with individual line selection buttons for each available telephone line.
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