In the early days of the telephone (early to mid-20th century), access to this new technology was more common in large cities and less so in remote areas. When telephone service was extended to rural areas, phone companies often employed party lines that could be shared between as many as 20 homes to minimize the number of lines they needed to string (and maintain).
Party lines worked liked this: Every household that was part of a given party line had its own phone number and a unique Morse Code-like ring, such as four short rings or a long ring, short ring, long ring. If your mother was calling you, the phone would ring in everyone's home that shared your party line. But you'd know the call was for you because you'd recognize your "ring." Similarly, the other members of the party line would know to ignore the call because it wasn't intended for them [source: The Telecommunications History Group].
While you could call members of your party line by yourself, any calls you wished to place to people who were not on your party line had to go through a switchboard operator, who would connect your call. But with the advent of advanced phone technology, it became just as simple to establish one-person lines as party lines. With today's computer technology and voice recognition software there is seldom a need for a human to operate a switchboard at all. However, the jobs still exist at hospitals and some large corporations.