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Party line systems were widely used to provide telephone service starting in the 19th century. By the mid-20th century a majority of telephone subscribers in the Bell System in the United States and Canada were serviced by party lines, which carried a billing discount over individual service when available. British users similarly benefited from the party line discount. Farmers in rural Australia used party lines, where a single line spanned miles from the nearest town to one property and on to the next.
Telephone companies offered party lines since the late 1800s, advertising the service in newspapers of general circulation since at least 1897, although subscribers in all but the most rural areas may have had the option to upgrade to private line service at an additional monthly charge.
Party lines were widely used in Australia where sparsely populated remote properties are spread across large distances. These were operated by the Government Post Master General department.
In rural areas in the early twentieth century, additional subscribers and telephones, often numbered in several dozen, were frequently connected to the single loop available. Party lines provided no privacy in communication and were frequently used as a source of entertainment and gossip, as well as a means of quickly alerting entire neighborhoods of emergencies such as fires, as written in The Thorn Birds by Colleen McCullough. As such, they became a cultural fixture of rural areas for many decades.
Wartime rationing in WWI and WWII of strategic metals, such as copper, required telephone companies to connect multiple subscribers to one line. This often led to traffic congestion in the telephone network, as the line to a destination telephone was often busy. Nearly three-quarters of Pennsylvania residential service in 1943 was party line, with users encouraged to limit calls to five minutes. Shortages persisted for years after each war; private lines in Montréal remained in short supply at the end of 1919 and similar shortages were reported by telephone companies in Florida as late as 1948, Some rural users had to run their own wires to reach the utility's lines.
In December 1942, University of Tennessee's strategy in a US football game versus University of Mississippi was revealed to the opposing coach as a telephone on the Ole Miss team's bench had been inadvertently wired to the same party line. In May 1952, an alleged bookmaking operation in St. Petersburg, Florida was shut down after one month of operation in a rented storefront using a party line telephone. In June 1968, the conviction of three Winter Park, Florida men on bookmaking charges was overturned as police had used a party line telephone in a rented house on the same line as the suspects to unlawfully intercept their communications.
While primitive lockout devices to prevent two subscribers from picking up the same line at the same time were proposed relatively early, multiple simultaneous calls did not become viable until the initial tests of transistorised pair gain devices in 1955. Any handset off-hook therefore tied up the line for everyone.
Many jurisdictions require a person engaged in a call on a party line to end the call immediately if another party needs the line for an emergency. Such laws also provide penalties for abuse by falsifying emergency situations. In May 1955, a Poughkeepsie, New York woman was indicted by a grand jury after her refusal to relinquish a party line delayed a volunteer firefighter's effort to report a grass fire; the fire destroyed a shed and a barn. She was given a suspended sentence. In June 1970, a sixteen-year old girl and a woman were charged after refusing to relinquish a party line to allow a distress call as three boys drowned in a pond in Walsenburg, Colorado.
Objections about one party monopolizing a multi-party line were a staple of complaints to telephone companies and letters to advice columnists for years and eavesdropping on calls remained an ongoing concern.
By the 1980s, party lines were displaced in most localities as they could not support subscriber-owned equipment such as answering machines and computer modems; the electro-mechanical switching equipment required for their operation was supplanted by electronic switching systems. The new telephone exchange equipment offered vertical service code calling features such as call forwarding and call waiting which was incompatible with party line technology.
In 1971, Southern Bell had announced plans for phase-out of party lines in North Carolina. In 1989, the Chesapeake and Potomac Telephone Company replaced party lines with private lines in Talcott, West Virginia, a rural area which once had as many as sixteen subscribers on one line. In 1991, Southwestern Bell set out to replace all of its party lines in Texas with private lines by 1995. Woodbury, Connecticut's independent telephone company abandoned its last party lines in 1991, the last in that state to do so.
Party lines in the United States were ineligible for Universal Service Fund subsidies and telephone companies converted them to private lines to benefit from the subsidies. Universities also phased out the systems, which were once common in student dormitories. Illinois State University terminated its last party line in 1990.
One of the last manual telephone exchanges with party lines in Australia was closed down in 1986 in the township of Collarenebri, where most town residents had a telephone number of only three digits, and to make a call outside the exchange area it was necessary to call the exchange to place a call. For rural residents, many were on a single telephone line identified by a number and a property name, for example one party line was called Gundabluie 1 line. Each party on that single line was identified by a letter, and so to call that party, the exchange would be called and the number asked for would be Gundabluie 1 S for example. The exchange rang a distinct ring down the Gundabluie 1 line, signalling the party's corresponding letter in Morse code. This distinctive ring would alert all parties on the line who the call was for. Three short rings signified the call was for the party with the S letter and so on.
Originally, in order to distinguish one subscriber from another on a party line, operators implemented a different ringing cadence for each subscriber, such as two short rings, or one short ring followed by a long ring. Each subscriber heard the ringing sequence and recognized their own to answer the call. Subscribers were able to listen in on other subscribers' calls. Frequently ringing phones were an annoyance, and more selective ringing methods were introduced using various technologies.
Especially effective on two-party lines was the distinction between tip party and ring party. Each telephone bell, rather than being connected across tip and ring as usual, was connected from one wire to local ground. Thus only the selected station in a two party line would ring. For multiparty lines all the "tip parties" or all the "ring parties" would ring, in this semi-selective scheme. This system was also used in the United Kingdom where X and Y subscribers on an A&B wire system would be rung on B wire and earth for the X subscriber and on the A wire to earth in the case of the Y subscriber. The momentary earth condition to initiate a call by first getting dial tone would have a similar convention. Frequently, only sets that are owned and installed by the connecting telephone company are permitted on the party line; the phone's bell had to be configured and grounded by a technician for the appropriate ringing signal. An off-the-shelf telephone may ring for both lines if plugged into the party line.
Later, independent systems applied multiple ringing frequencies for fully selective ringing. (The Bell System eschewed frequency selective ringing.) The ringers in party-line phones were tuned to distinguish several different ringing signals so that only the desired party's phone would actually ring. In this arrangement the only inconvenience of a party line was occasionally finding the line in use (by hearing talking) when one picked up the phone to make a call. If one of the parties used the phone heavily, then the inconvenience for the others was more than occasional, as depicted in the 1959 comedy film Pillow Talk.
Even for lines with selective ringing, calls to another party on the same party line, known as "reverting calls", required special equipment and procedures.
In the local-battery system of the early cranked magneto phones, the phone's own battery powered its transmitter as well as the receiver of the called phone. If too many phones were off-hook and listening, the additional receivers would load down the transmitter's battery with a voltage so low that no phone could receive an intelligible signal.
With party-line service, particularly if there were more than two subscribers on the line, it was often necessary to complete a long-distance call through the operator to identify and correctly bill the calling party. In some cases, the calling party would misidentify themselves in an attempt to send the bill to another party.
A two-party line split between tip party and ring party could be created in such a way as to allow the central office to determine which party placed an outbound toll call by detecting that one of the ringers was disconnected when that subscriber went off-hook. This system would fail if any provision was made to allow the subscriber to turn off the bells (do not disturb) for privacy or unplug the telephone; it also presumed that each subscriber only had one telephone connected to the line.
Systems which identify the caller's name and address to emergency telephone numbers (such as Enhanced 9-1-1 in North America) may be unable to identify which of multiple parties on a shared line placed a distress call; this is aggravated by the use of old mechanical switching equipment for party lines as this obsolete apparatus consistently provides no caller ID and often also lacks automatic number identification capability.
When the party line was already in use, if any of the other subscribers to that line picked up the phone, they could hear and participate in the conversation. Eavesdropping opportunities abounded, as shown in the 1959 film Pillow Talk.
Mischievous teens soon discovered that calling their own number and hanging up would make all phones on the network ring, and many of the residents on the system (sometimes a half a dozen or more) would answer the phone at the same time.
Party lines were typically operated using mechanical switching systems which recognized certain codes (such as "dial 4101 and hang up" to ring a phone on the same line) which no longer work on modern electronic switching systems or digital switchgear.
Party lines are not suitable for Internet access. If one customer is using dial-up, it will jam the line for all other customers of the same party line. Bridge taps make party lines unsuitable for DSL, even in the few areas where distance from the central office does not already preclude its use. Telephone companies typically do not allow client-owned equipment to be directly connected to party lines, posing an additional obstacle to their use for data.
Party line usage was at one time common on railways, where numerous telephones were connected to a single pair of wires. Usually a long ring of many turns of the handle would alert the exchange that a connection was required to another destination. The problem of selective calling was also solved by a mechanical device which could selectively ring one or a group of stations.
Modern telephone systems do in some cases use one copper pair as a shared medium on which to carry telephone service to multiple subscribers in a distant location. Various pair gain schemes allow time division multiplexing or frequency division multiplexing so that multiple calls are carried simultaneously on the same pair without interference. A distant suburb may have a subscriber loop carrier or digital loop carrier system in which cabinetised equipment is located near the subscribers to connect the multiple, individual phones to one common line back to a central office exchange. A single optical fibre can also be shared between multiple subscribers in fibre to the cabinet systems.
Digital wireless connections (such as cellular telephones or voice-over-IP running over rural wireless Internet) are also inherently a shared medium. A sufficiently high usage of simultaneous active connections will cause a mobile telephone network to become congested or voice transmitted by Internet telephony to break up unintelligibly. Operated within their capacity, these systems do give the illusion of a virtual private line as calls may be carried without mutual interference.
Party lines remain primarily in rural areas where local loops are long and private loops uneconomical when spread sparsely over a large area. Privacy is limited and congestion often occurs. In isolated communities, party lines have been used without any direct connection to the outside world.
One example of a community linked by party line is in Big Santa Anita Canyon high in the mountains above Los Angeles, near Sierra Madre, California, where 81 cabins, a group camp and a pack station all communicate by magneto-type crank phones. One ring is for the pack station, two rings for the camp and three rings means all cabins pick up.The system is a single wire using the ground as a return path; as of 2012 it remained partially functional but poorly maintained.
In modern use, the term "party line" has occasionally been used to market conference bridge and voice bulletin board service, but these are not party lines in the original sense of the term as users call in using multiple, individual lines.