Introduction to UK phone sockets and stuff.....

The UK Telephone socket - viewed from internal circuit board side.

UK Telephone Master socket
Master Socket

UK Telephone Slave socket
Slave Socket

Interconnecting wiring between Master and Slave sockets
UK Telephone socket interconnection (diagrammatic representation)

The Master Socket will be wired up by the telephone company (in UK this is usually BT). To connect additional Slave (extension) Sockets follow the diagram above. Whenever possible use proper single-core telephone wire and connect according to the colours shown.

The connections will either be screw terminals or IDC (Insulation Dispacement Connectors). A special tool is required to push the wires into the IDC metallic contacts. This can usually can be purchased in the same store as the cable.

The UK Telephone socket - circuit operation.

UK Master socket internal circuitry
Master Socket - circuit diagram

To understand the circuit operation of the Master socket it is necessary to understand the voltages present on the telephone line.

  • The power for the speech circuit is 50 volts DC.
  • The power for the ringing (bell) circuit is 75 volts AC.

The telephone line comes in on Pins 2 and 5, and connects to the speech and dialling circuitry within the phone handset (on all of the sockets).

The ringer/bell circuit in the phone handset is connected between Pins 3 - 5 (keeping it separate from the speech/dialling circuitry). Capacitor 'C' couples the AC ringing current from the exchange on pin 2 to Pin 3 on all the sockets while at the same time it blocking the DC of the speech circuit from entering the ringer.

Each ringer is assigned a REN number (Ringer Equivalent Number). The UK phone network will support a maximum REN of 4. The REN value is normally marked on each phone. Phones will not ring reliably if the total REN on an installation exceeds 4.

Component 'SA' is the spark arrestor which is designed to protect the phone from high voltage spikes (eg lightning).

Resistor 'R' is permanently in the circuit for testing purposes if the phone is unplugged.

Telephone cable - wire colours
Telephone wire colours

In this example WHT/GRN and GRN/WHT

The first mentioned colour is the background colour, the second colour is the stripe.

Wiring Colour Codes
Old BT
Current BT
  BLU     Blu/Wht
  ORG     Wht/Blu
  GRN     Org/Wht
  BRN     Wht/Org



Connecting diagram for older phones
Connect an older telephone to plug-in sockets
as per this diagram.

To connect an older phone (GPO Type 300 or 700-series) to the newer style of plug-in socket a new cable complete with fitted plug has to wired into the internal terminal strip.

The terminal strip has to be wired as shown in this diagram (noting the position of the straps between various terminals).

If connecting a TrimPhone remove the strap between T4 - T5 and replace it with a 3300 ohm resistor.

For more information on converting old phones to the plug and socket system go here



Dialling a call

Rotary Dial
The rotary dial mechanism was patented in 1923 by Frenchman Antoine Barnay. Selecting a number and rotating the dial with a finger winds up the internal mechanical mechanism.

UK rotary dial
UK telephone dial c.1970

Rotary dial pulse-wheel and contacts
Dial pulse wheel and contacts

Once released, the dial un-winds at a constant speed. The rotating motion causes a toothed wheel (the pulse wheel) to repeatedly open and close an electrical contact wired directly into the telephone line.

The making and breaking of the circuit is detected at the exchange as a series of pulses corresponding to the number dialled (number 0 [zero] creates 10 pulses).

DTMF (Dual Tone Multi-Frequency)
In the early 1960's AT&T introduced the Touch-Tone® phone.


Telephone keypad
Telephone keypad (red zone shows the
normally unavailable extra buttons)

The conventional dial of the telephone was replaced by a 12 button keypad. When a key is pressed the tone of the column (High Tone) and the tone of the row (Low Tone) are simultaneously generated (eg, pressing the '5' button generates the tones of 770Hz and 1336H). This dual tone is sent down the telephone line to the exchange.

The tones were chosen to avoid harmonics (no tone/frequency is a multiple of another, the difference between any two tones doesn't equal any of the frequencies, and combining two tones does not equal any of the other tones). The frequency, amplitude and time duration of each dual-tone is defined in various international technical specifications (eg CCITT).

The A, B, C and D buttons are not available on ordinary phones. They were originally used by the US military, and are now used for signalling on the telephone circuits between exchanges.

The "#" key is known by several different names including Hash, Pound, Crosshatch, etc.



Mobile (Cellular) phone stuff  

Mobile Phone security info.
Punch in   *#06#   - then write down the displayed 15-digit security number. This is called the IMEI Number.

If your phone is stolen quote this number to your Network Provider. The stolen phone will be permanently disabled.

If your phone gets stolen call 08701 123 123 to have it switched off
                               (Disclaimer: uncletony hasn't personally verified this number).


Phone charges  

How much will that phone call cost?
For a comparision of charges between UK telephone providers go here

Carrier Pre-Selection (CPS)
In UK it is now possible to select the provider (or carrier) of your telephone service. This has led to a general reduction in the cost of telephone communications. No adaptors or special dialling codes are required. Once the required documentation is completed all your calls (or selected types of calls) will be routed to your new service provider.

It is important to be aware that although another provider will be charging you for your calls, BT still remains the owner and operator of your phone line and will bill you for this.

For an up-to-date guide to CPS, visit the Ofcom website, here.

A movement in UK calling themselves "SAYNOTO0870.COM" exposes the high costs of 'National Rate' calls (which use the 087xx number group). Go here and be suprised at the costs involved - and get very annoyed!!


How to get rid of those annoying sales calls ....

Register with the Telephone Preference Service and get no more sales calls

Do you get loads of double glazing and kitchen salesmen phoning?
   Register your number (land-line and mobile) with TPS and within a few weeks no more sales calls.

Do you get loads of junk mail?
   Register your address with MPS and within a few weeks the junk mail will dry up.

These are totally free and fully legal services supported by the Direct Marketing Association.


Register with the Mail Preference Service and get no more junk mail

Also available on either of these sites is an option to Register your Fax number (to stop unsolicited advertising sent by Fax) and Register for the BabyMPS service (designed to prevent harrassment from companies offering services related to your baby).

... and who to Complain to.
Since December 2003 it has been illegal (in UK) to send unsolicited text marketing messages to mobile phone users. To complain about receiving such texts contact the Office of the Information Commissioner (01625 545 745). If the text message contains a premium-rate phone number then also contact ICSTIS, the Independent Commission for the Supervision of Standards of Telephone Information Services regulator (0800 500 212).

To block automated calls that are often silent contact Silent Callgard Service (0870 444 3969)

OTELO is the Office of the Telecommunications Ombudsman (UK only)- take unresolved complaints about telephone bills here.

Contact ICSTIS if you've been a victim of a computer Trojan/virus that has 'taken over' your regular ISP dial-up number unknowingly (until you get the phone bill!) and replaced it with a very expensive Premium Rate number (those beginning with 09xx). ICSTIS logo


For a very interesting historical account of UK telephones, pictures of phones and phone equipment
plus technical info visit telephonesuk.co.uk created by Mike Fletcher here

To check the time call Mike's very own TIM - 0870 789 3642 (UK local call rate)

If you wish to see telephone information from other countries included please send details to uncletony

e-mail uncletony


Telephone Directories
For UK numbers go here: www.bt.com

For worldwide numbers go here: www.infobel.com/teldir/ or here: www.anywho.com

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