British telephone socket
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, 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', which could be very complicated and required the attendance at the premises of a GPO telephone-engineer, who needed a complete set of 'N' Diagrams, which was very extensive and ran to over 15 volumes of little black ring binders. N diagrams also had their own numbering system, and were frequently updated.
From the early years of the 1900s, the GPO had a plug and socket system available for rent. It was later called a "Plan 4", and employed a heavy-duty, four-way jack plug 404, 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 on each telephone and on all the sockets, and hence the system was not that common.
Sockets
A domestic single British telephone line installation will have a single master socket or line box in the premises, which is provided by BT or another service provider: this socket is the demarcation point between the customer-owned and maintained on-premises wiring, and the telephone network. For installations using the NTE5 line box, the demarcation point is actually within the socket: the lower half of the front plate and associated wiring is the customer's, while the permanent wiring on the non-removable section behind this, remains the responsibility of the service provider. Customers are not permitted to access the wiring in a master socket without a removable lower section. Plug-in extension kits are available for customers with this type of installation. The two wires from the exchange are denoted the B leg at −48V relative to ground when the line is not in use and the A leg which is near ground potential when the line is not in use. The A leg goes to pin 5 and the B leg to pin 2 in the master socket. When current is flowing on the line, the B leg voltage collapses to nearer ground and the A leg voltage moves nearer to the B leg voltage. The exact voltage drop is a function of the distance to the exchange, and the network wiring type.According to SIN 352 the average DC current in the loop and voltage across the phone will be up to 42 mA at 12.5 V, up to 33.5 mA at 10 V, and will be not less than 25 mA at 9 V.
Line polarity reverses during calls if caller ID is in use.
Once in a call the audio and tone levels superimposed on the DC voltage are expressed as dBm in 600 ohms, although the line impedances are permitted to be some way off 600 ohms -9 dBm 0 dBm = 1 mW in 600 ohms.
Until recently, this socket contained an enclosed [spark gap, SP1, that could safely flash over internally to provide high voltage surge protection. This component is no longer used due to negative effects on VDSL speeds. The socket includes a 1.8 μF capacitor to feed the AC ringing and a 470 kΩ resistor to permit remote testing when no telephones are plugged into any sockets. Additional internal extension sockets are wired off the master socket but not containing the surge protector, bell circuit capacitor, and the out-of-service resistor.
The 'old style' fixed master socket had only one set of terminals on the back and customers were supposed to use extension kits plugged into the front socket; however, many customers hard-wired their own extensions anyway for neatness and robustness reasons, which was a poor arrangement since it provided no way to isolate the customer's internal extension wiring from BT's wiring.
NTE5 Linebox sockets have for many years been fitted in place of master sockets. The lower part of the front plate can be removed after unscrewing two screws, allowing users access to the terminals required for connecting internal extension sockets. The removable panel also allows the external telephone line to be easily disconnected from the internal wiring, provided the wiring of the premises has been correctly carried out. This leaves a single working master telephone connector; if a telephone connected to this connector works while there had been problems with telephones connected to extensions, faults must be due to the customer's wiring in the building. The terminals on the back part were originally large screw terminals, later replaced in all BT NTE5 sockets by insulation-displacement connectors.
As BT no longer has a monopoly of internal wiring, they make a substantial charge if a fault reported to them is found to be in the customer's internal/domestic wiring. It is therefore important for the customer to have the facility to check whether a fault is in their internal wiring/equipment or externally in BT's cabling or systems. Since the NTE5 socket represents the official demarcation point between the internal/domestic wiring and the external telephone line/cabling fixed at the rear the physical disconnection of the two sets of wires is crucial in identifying faults and allocating responsibility for their rectification.
In 2009 BT Introduced a vDSL service to the UK known as BT Infinity, and at the same time introduced the BT vDSL Interstitial Faceplate, which performs two functions: DSL filter and Bell Wire noise suppression. The vDSL modem now plugs directly into the 6P6C modular socket on the faceplate. The faceplate can be easily fitted by removing the two bottom screws on a NTE5, sliding the bottom section out and fitting this in between. The result is that the entire extension circuit is filtered by the vDSL plate, so that DSL filters are no longer needed on a telephone socket used for DSL. The vDSL Interstitial Faceplate can also be fitted to lines used for ADSL, for which it has been shown to improve connection speeds.
There are several versions of the NTE5 and of the vDSL Interstitial Faceplate.
Plugs
| Pin | Pair | Signal | Color |
| 1 | Reserved | Green/white | |
| 2 | 1 | Leg A | Blue/white |
| 3 | External Bell wire | Orange/white | |
| 4 | Reserved | White/orange | |
| 5 | 1 | Leg B | White/blue |
| 6 | Reserved | White/green |
There are two types of modern British Telecom plugs in common use for connecting telephones, the 431A and 631A.
431A is 4-way and 631A 6-way. There are also plugs with only two contacts commonly seen on modem leads. These are a recent introduction and do not seem to be easily available as separate parts. All fit any right-handed "Type 600" telephone socket.
Type 430A and 630A plugs have the latch on the opposite side of the plug, and were used as headset plugs on some switchboards and as handset connectors on some telephones, e.g. Ambassador.
The 631A and 630A plugs are also used for connecting sensors to interfaces for computer-based measurements in educational environments, the former for connecting analogue sensors and the latter for digital sensors. Companies using these plugs include Vernier, TI and Casio, for interfaces connecting to their graphical calculators, and in the Netherlands CMA.
The BS 6312 specification defines the terminal numbering of a socket-outlet in the opposite direction to the pin numbers of the socket. Thus terminal 1 is connected to pin 6, terminal 2 to pin 5 and so on. The pins of the 631A plug are numbered in ascending order from left to right with the contacts facing upwards and the latch on the right-hand side.
Connector on phone
The connector on the phone is not standardized: the connector at the wall is standardized by regulation, to allow individuals to use their own phones, but the wire from the phone to the wall may be hard-wired to the phone, or use various connectors.Typically it will have a 6P4C or 6P2C modular connector at the telephone end: this latter may be wired as per the RJ11 standard, or it may be wired with pins 2 and 5, as a straight-through cable from the BT plug. Thus cables are not in general compatible between different phones, as the phone base may have a socket with pins 2 and 5, or have an RJ11 socket.
Use in other countries
The BS 6312 jack has been used in New Zealand since the 1980s, replacing a number of other connectors and hard-wired connections, and was subsequently replaced by a "2-wire" version suited to daisy chain wiring that eliminated the 3rd anti-tinkling wire. The "BT Jack" is still the most common phone jack in use, although many installations in business use structured cabling with "RJ45" 8P8C modular connectors for telephone as well as data services. Since 2010 the Telecommunications Carriers Forum Premises Wiring Code of Practice has deprecated BT jacks in favour of "RJ45" modular jacks for all new residential and SOHO phone/data networks, although not yet a mandatory standard in 2016.It is also used in Bahrain, Bangladesh, Belize, Botswana, Brunei, Cyprus, Eritrea, Eswatini, the Falkland Islands, Ghana, Gibraltar, Israel, Jordan, Kenya, Kuwait, Lesotho, Malawi, Malta, Myanmar, Nigeria, Oman, Qatar, Saudi Arabia, Tanzania, the United Arab Emirates, Zambia, and Zimbabwe. The jack is still found in Hong Kong, where new installations ceased in 1998, while in Saint Vincent and the Grenadines, new installations ceased in 2001, with RJ11 now used instead.
While BS 6312 was not adopted in the Republic of Ireland, Malaysia or Singapore, Irish, Malaysian and Singaporean phone jacks bear some varying levels of similarity to their UK cousins due to GPO influences in the old Irish, Malaysian and Singaporean phone wiring systems. The Malaysian Telecommunications Department and Telecommunication Authority of Singapore adopted Bell System-style 6P2C plugs and 6P4C "RJ11" sockets in the 1970s, with Ireland's P&T doing so slightly later, some time before BS 6312 was rolled out by BT in the UK. Irish, Malaysian and Singaporean phone sockets are normally wired, as per the international standard, with the line carried on the centre pair. However, although rarely connected in practice, Irish phone jacks also contain a ringing capacitor circuit very similar to their UK counterparts. This is carried on pin 5. Or, if wired for two-line service, a second ring wire is carried on pin 2 for line 2, with the outer pair, pins 1 and 6 carrying the second line. This arrangement was introduced for the same reason as the capacitor in BS 6313; to allow backwards compatibility with older GPO style type 3 wire phones that lacked an anti-tinkle circuit, which were common in the 1970s and into the 1980s. Modern Irish jacks also contain screw down lugs to connect extension phone wiring or a hardwired pre 1970s phone. The connection block has an "R" terminal for connecting the ringing wire to the capacitor. Despite the circuitry being available in the jack, UK phones should still be connected with an adapter with its own ringing capacitor, as the "R" wire typically is no longer connected in most homes, unless the wiring was originally used with rotary dial telephones.