Wiring And Repairing Telephone Sockets


Not everyone knows the difference between an extension telephone socket and a master socket, especially if it isn’t something you’d normally deal with.  Extension telephone sockets are normally wired from the master socket but, more recently, NTE5 or CTE5 Line boxes have replaced the master sockets. They are fitted with removable panels which are used to house the terminals and connect the wiring to secondary sockets.

Simply put, the main differences are:

  • A primary or master socket contains three main components an extension socket doesn’t have.

The components are:

  • A surge suppressor across the line pins 2-5
  • A capacitor coupling the AC ring current running from one side of the line (pin 2) to pin 3
  • A high-value resistor which forms a return pathway from pin 3 to the other side of the line (pin 5). This means that line testing equipment knows there is a termination which has nothing plugged into it.

Tools Needed

  • A Punch Down Stripping Tool – To secure seating of data wires quickly into terminals, automatic, precise trimming and sealing action.
  • Slotted Parallel Flat Head Insulated Screwdriver – For example, the C.K Dextro VDE Screwdriver Slotted Screwdriver provides extra response and more speed. It’s comfortable to work with, ergonomically designed and has a rounded spinning area.
  • Screwdriver – the C.K Tools Robertson Dextro Screwdriver, for example, is designed by professionals, again ergonomically designed.
  • Self Driving Wood Screws – To secure your installations into place.
  • Telephone Master Socket Flush Wall Switch – this is designed to connect your property to the service provider, whereas the  Secondary Sockets is for general use around the house.
  • ADSL Broadband Faceplate Splitter Adaptor Socket –  This is to increase your broadband speed and reliability and replaces the lower half of your BT NTE5a or NTE5b master socket. It provides outlets for RJ11 (for ADSL modems) and 431A (for telephone line equipment).
  • ADSL Filtered BT / RJ11 Phone / Network Faceplate – For anyone who wants an integrated ADSL microfilter into a faceplate with a BT and an ADSL-RJ11 socket. It eliminates the need for a plug-in style filter which just gives a more polished finish.

Telephone Socket Wiring Tips

  • On older socket installations (prior to 1980) grey or cream cables were used which had conductors coloured blue to pin two, brown to pin three, green to pin four (if it’s to be used) and orange to pin five. At times the green would be used for pin three and brown for pin four.
  • On units of internal extension wiring that were produced after BT/Openreach went back to using white four core cable with solid colouring: blue to pin two, brown to pin three, (if used) green to pin 4 and orange to pin 5.
  • Extra Bells are wired between pin three which is orange and white and pin five which is white and blue. Outside overhead telephone line colour coding is as follows: orange/white is line one; green/black is line two.
  • The BT Drop Cable (The cable coming from outside) often has Orange, White, Green and Black wires. In many cases, orange and white are the active cables and attach to connections 2 and 5 in the master socket. In some master boxes, they go to two connectors marked A and B.
  • It isn’t important which way round they are connected but there are modems and answering machines which are specific about polarity, so you need to check the voltage on the line then connect -48V to your Aleg (5) and 0V to your Bleg (2) inside the master socket.
  • If your wiring is underground and has a small grey connection box by your door, the internal cabling will normally be the same type and colour as the extension cables.

Typical Phone Line Faults

When it comes to faults within the units they can be difficult to find as they’re not always obvious.  The main faults will be wrong or bad connections here are a few of the most common faults:

  • Lack of ringtone – Potentially a disconnection of Terminal 3.
  • Constant ringtone – Terminals 5 and 2 swapped (5 at one socket connected to 2 on another and vice versa).
  • Bad quality of speech or even a bad bell – Terminal 3 and 2 transposed.
  • Phone ringtone but poor or no speech and unable to dial out. – Wiring between terminals 2 or 5 damaged or broken.

Testing your Cabling

If your main socket is working but the remote socket isn’t then tested the continuity of your circuit. You could use a very long lead and a test meter – or you could cheat!

  • Disconnect your BT line completely.
  • Bridge any 2 terminals at the remote end and take note of which two you’ve bridged.
  • Measure the continuity between these two wires at the master socket end – it shouldn’t be any more than a few ohms.
  • Repeat the process for the second pair of wires.
  • If either connection shows faults then swap the combinations – if you’ve tried 2 and 4 and found them OK, then tried 3 and 5 and it failed, you then know that 2 and 4 are good so trying 2 and 5 and 2 (or 4) and 3 will show up your fault.

So, now you’ve tested your cables but there is a cable buried, for example, in a newly decorated wall with continuity in only 2 wires, what happens then?

In this case and only then, you cheat by using an extra master socket.

This second master socket will give you back your ringtone. Connect your two working wires to terminals 2 and 5 on this new master.

Ringer Equivalence Number (REN)

REN is a measurement of the+ load a device places on the line when it rings. The average BT line will support at least 4 REN (4 phones/modems). The number of REN added together shouldn’t ever be more than 4. (You’ll usually find the REN on a label on the bottom of the machine).

It is easy to exceed the number simply because devices which have a REN of 1 could actually have a REN of a fraction of 1 which is inconsistent with the test procedure used and, a lot of lines can drive a REN of more than 4.  You can buy a REN Booster which can increase the ringing capacity of a line, but if you’re at that stage you should be considering installing a small PABX.


Telephone circuitry hasn’t changed that much over time but with the introduction of the internet, the outlets and configuration needed to change to facilitate this. Adding a router/modem to your phone line filters to stop ‘noise’. A plug-in filter was a common way to fix this but they were susceptible to bangs, knocks, and failures.


Now, manufacturers have developed faceplates providing outlets to suit your requirements.  These are designed to replace the need for microfilters on telephone extensions and filter the signal directly at the BT master socket, isolating the signal from extension cables throughout the building.

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Electrical World sell high quality electrical components to residential and commercial customers globally.

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Posted on 28th February 2019 at 7:52 pm

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