into the structured cabling via a line adapter unit (LAU) / Mod-Tap / Molex that contains the capacitor needed to connect the bell signal. Different types of LAUs are on the market: PSTN secondary/slave: for phones that need no bell wire PABX master: contains ring capacitor, to provide from the 8P8C 2-wire interface the 3rd bell wire required by UK phones PSTN master: implements all the circuitry found in a BT master socket, including surge protection, ring capacitor, test resistor There exists no well-established standard in which polarity pins 2 and 5 of the phone socket are connected to pins 4 and 5 of the 8P8C connector; both alternatives are common. This polarity is not normally important; BT does specify that the B wire is more negative than the A, though they no longer specify which of the A and B wires are connected to pins 2 and 5 of their master telephone socket, although earlier standards specified that the A wire be connected to 5 and the B wire to 2. See also RJ11, RJ14, RJ25 Technical standards in Hong Kong, influenced by British standards as introduced during British rule GPO telephones Notes External links Wiring diagram UK plug and socket telephones Using ADSL with extension telephone sockets The RJ System of telephone plugs and sockets is gradually replacing the BT System in Hong Kong, Office of the Telecommunications Authority (OFTA), Hong Kong Special Administrative Region Government BT Group Telephone connectors Telecommunications equipment