British telephone sockets were introduced in their current plug and socket form on 19 November 1981 by British Telecom to allow subscribers to connect their own telephones. The connectors are specified in British Standard BS 6312. Electrical characteristics of the telephone interface are specified by individual network operators, e.g. in British Telecom's SIN 351. Electrical characteristics required of British telephones used to be specified in BS 6305. They are similar to modular connectors (as used in RJ11), but have a side-mounted hook, rather than a bottom-mounted one, and are physically incompatible. History Standard sockets were introduced, as part of the 'New Plan' wiring policy, to allow customers to easily purchase their own telephones, as required by Oftel, the phone regulator. Thus any phone whose plug conformed to BS 6312 and met certain other regulatory standards, such as BABT, could be connected to the network, rather than British Telecom controlling the market. The 'New Plan' was only new to the UK and was based extensively on systems which had been available elsewhere for many years, especially in the US. The new system replaced the older hard-wired system, which came in many 'flavours' (e.g., Plans 1, 1A, 1B, 1C, 2, 2A, 105, 107 etc.), which could be very complicated and required the attendance at the premises of a GPO telephone-engineer, who needed a complete set of 'N' (wiring) Diagrams, which was very extensive and ran to over 15 volumes of little black ring binders. N diagrams also had their own numbering system (e.g., a Plan 1A had an N diagram of N4502), and were frequently updated. From the early years of the 1900s, the GPO (subsequently British Telecom) had a plug and socket system available for rent. It was later called a "Plan 4" (N762—first edition), and employed a heavy-duty, four-way jack plug 404 (circular in cross-section), on the end of the standard, plaited, cotton-covered instrument cord. It also had to have a separate bell-set, which was permanently in-circuit to provide ringing if there were no telephones plugged in. This system survived through various models of telephones from the "candlestick" 200 and 300 type Bakelite phones until the introduction of the 700 series in 1959, when a smaller "Plug 420" was introduced. The separate bell-set, with its on-board capacitor and coils, also provided a testing circuit for remote engineers, by providing the mandatory 1000 ohm capacitive loop-back. Rental had to be paid