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A telephone plug is a type of male connector used to connect a telephone to the telephone wiring in a home or business, and in turn to a local telephone network. It is inserted into its female counterpart, a telephone "jack", commonly affixed to a wall or baseboard. The standard for telephone plugs varies from country to country, though the RJ11 "modular connector" has become by far the most common.
A connection standard, such as "RJ11", specifies not only the physical connector, but how it is wired (the "pinout"). Modular connectors are specified for the Registered Jack series of connectors, as well as for Ethernet and other connectors, such as 4P4C (4 position, 4 contacts) modular connectors, the de facto standard on handset cables, often improperly referred to as "RJ" connectors.
Historically the telephone was often owned by the supplier and permanently wired in to the telephone line they supplied but as phone markets became more deregulated there was a need for a simple plug-in interface that consumers could use. Many countries initially used their own connectors. For example Bell System companies in the 1960s used a round plug about 40mm in diameter with four prongs about 15mm apart. National connectors remain in service but few are used for new installations.
On a conventional wired telephone, there are 4 connections, each of which may be hardwired, but more often uses a plug and socket:
telephone line to phone cable
The wall jack. This connection is the most standardized, and often regulated as the boundary between an individual's telephone and the phone network. (In many homes, though, the boundary between utility-owned and household-owned cable is a jack on an outer wall; all wall jacks in the home are part of the household's internal wiring.)
phone cable to phone base
This and further connections are generally not regulated, but instead have de facto standards. It is often 6P4C, which is often RJ11, but may be proprietary or hardwired.
phone base to handset cable
handset cable to handset
The last two (the handset cable) has a de facto standard of a 4P4C connector with straight through cable.
Some of these may be absent:
A standard specifies both a physical connector and how it is wired. Sometimes the same connector is used by different countries but wired in different ways.
For example, telephone cables in the UK typically have a BS 6312 (UK standard) plug at the wall end and a 6P4C or 6P2C modular connector at the telephone end: this latter may be wired as per the RJ11 standard (with pins 3 and 4), or it may be wired with pins 2 and 5, as a straight through cable from the BT plug (which uses pins 2 and 5 for the line, unlike RJ11, which uses pins 3 and 4). Thus cables are not in general compatible between different phones, as the phone base may have a socket with pins 2 and 5 (requiring a straight through cable), or have an RJ11 socket (requiring a crossover cable).
When modular connectors are used, the "latch release" of the connector should be on the "ridge" side of flat phone wire in order to maintain polarity.
Though four wires are typically used in U.S. phone cabling, only two are necessary for telecommunication. In the event that a second line is needed, the other two are used.
Different telephone connections are generally compatible with the use of an adapter: the physical connector and its wiring is the primary incompatibility.
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