How do phones work?

Okay, so tell me this. How do phones work ?

A very good question, that one, and for the moment the only answer I can direct you to is the file in the comp.dcom.telecom archives at []. It is US orientated, but some of the general principles remain the same.

[A UK version of this file would be gratefully received]

Where can I get technical information about UK telecommunication systems?

You need Supplies Information Notes (SINs): dial 0800 318601. For a full technical report upon how BT's proposed Caller ID (CLID) service works, ask for SIN 227. You can also ask for the latest SIN index with lots of interesting stuff, and ask to be added to a mailing list for all new SINs. (SIN 227 may also be found at <URL:>).

What do those wires do?

[last update 27/3/95; Answers courtesy Alan J Flavell]

(Unless otherwise stated, this answer only deals with ordinary BT telephone lines using the British-type modular socket system.

N.B the wiring up to and including your master socket is the property of BT and the rules say you mustn't interfere with it. Therefore these details are here for educational value only.

British regulations prohibit the use of equipment that has not been type-approved for use in Britain. Therefore, any mention in here of equipment brought from elsewhere must be treated as purely hypothetical and for educational value only...)

At the point where your telephone line comes into the house, only two wires (1 pair) are used. If you have more than that, the remainder are spare. The wire costs almost nothing, compared to the cost of installing it, so the wire that they lay often has several pairs in it.

The wire pair does several things: it carries speech and bell-ringing current etc., and it brings a limited amount of DC power from the exchange; see the "How phones work" writeup for a general description, which is, however, expressed in US terminology.

The two wires of a telephone line pair are called A and B. These are connected to your master socket or linebox (everything said in here about a master socket is also true of a linebox, unless a specific difference is mentioned). Nominally, line A is at 0V and line B at -50V. {*}

(The 50V comes from a big battery at the exchange, which is kept charged from the public electricity supply ("the mains"). This is used to power ordinary telephones, thus allowing calls to be made even during mains failures. More sophisticated phones (e.g cordless) need a local mains supply, but the customer is advised to own at least one telephone that does not.)

Secondary sockets are connected to the master socket via two-pair or three-pair cable, in accordance with the instructions that accompany the kits. However, only three of these 4 or 6 wires are really used: namely, the two lines, and the bell wire (also known as "shunt", see later point).

The British telephone plug (type 431A) is rectangular: it has ways for six contacts along one side, and a latch (and keyway) on one end. Only the middle four contacts are actually present, with one blank position at each end, but the contact ways are numbered the same irrespective of whether pins are actually present. However, bizarre as it may seem, it turns out that there are two different schemes of numbering the pins: the original BT one, starting with pin 1 at the latch end, and the "BS" one, with pin 6 at the latch end. The physical wiring, thankfully, is the same in any event - it is only the contact numbering convention that is different.

 BT plug, contact face up,          pin number   installation wire
 with cable to the left              (BT) (BS)   colour (Body/stripe)

            no pin      not used      6    1      (White/green)
 .......... ---         line A        5    2       White/blue
 .......... ---         not used(E)   4    3       White/orange 
 .......... ---         bell shunt    3    4       Orange/white
 .......... ---         line B        2    5       Blue/white
            no pin      not used      1    6      (Green/white) 

There are some situations not covered by this FAQ (PABX Earth Recall for example) in which the pin marked (E) would be connected to a local earth.

The master socket contains the following circuit (SP=Surge Protection)

   ------------------------------o pin BT2 (BS5) ->
    line B     |      |
               |      = 1.8uF
 from          |      |
           SP  X      |----------o pin BT3 (BS4) -> to secondary socket(s)
 exchange      |      >         bell wire
               |      < 470kohms
    line A     |      >
   ------------------------------o pin BT5 (BS2) ->

In the normal UK telephone, the bell is connected between the bell wire and line A.

(There are some types of telephone approved for UK use in which the ringer is connected between the two lines, just as it would be elsewhere.)

(Many modems will be like this too, detecting ringing current from the line pair and disregarding the bell wire. If the modem also offers a socket for connecting a telephone, UK modems can be expected to supply the telephone with bell current correctly; but foreign modems might not do so, with the result that the phone, if it uses the "British" bell arrangement, will not ring.)

(There exists also a plug which is the mirror-image of the 431A, and which, of course, won't work in this socket: thankfully, most people will never meet one.)

Why a third wire for the bell?

The system was designed at a time when pulse (loop disconnect) dialling was common, and telephones still had real bells.

During pulse dialling, there is a tendency for the bells on the associated telephones to tinkle. By using a separate wire for the bell, it's possible for the telephone that's doing the dialling to shunt out the bells, preventing them from tinkling. Nowadays this is much less of an issue, what with tone dialling and electronic ringers.

What's this about master sockets and lineboxes?

A: The original design of "master socket" was wired by the installer, and the customer was not allowed to interfere with it. The approved way to wire a secondary socket, if you still have such an installation, is to purchase an approved extension kit which comes equipped with an adapter. After fitting the extension socket, you unplug the phone from the master socket, plug the adapter into the master socket in its place, and, assuming you still want to have a telephone at the master socket location, you plug it into the adapter.

The "linebox" is a much neater solution. It can be recognised by having a sub-panel on the front, which the customer can remove by undoing two screws. On removing it, one finds that the sub-panel is, in effect, an adapter, but with the advantage that the customer is allowed to punch down their own wiring to it. When investigating a fault, the engineer can remove the panel, together with the customer's wiring, and test the internal socket, free of confusion from customer additions. It is a good idea, before reporting a fault, to take off the subpanel yourself, and try a phone plugged into the internal socket in order to eliminate your own wiring - also try a second phone in case the first is defective - both kinds of fault would incur a charge if you call for a repair (line rental covers free repair only of BT's line).

There are, consequently, two different kinds of kit for installing a secondary socket. The one with an adapter is meant for the original "master socket"; it can be used with a linebox too, but it would be pointless to do so. The other kit has no adapter - only a length of telephone wire (two or three pair, it doesn't matter) and a secondary socket: this type of kit is for use only with a linebox.

My equipment (telephone, modem, computer...) has an RJ11 socket for connecting the line cord. What are the connections?

It is clear from the answers given by uk.telecom readers that there IS NOT ONE UNIQUE STANDARD. The two situations most likely to be met in practice are these

Some contributors asserted that one specific arrangement was ALWAYS used. But on the evidence, they must be mistaken.

Some contributors say that the first is more common, while others have more often met the second: so one can only guess what the statistics are.

At least one contributor implied that there are yet other pin assignments that have been used at the equipment end. (There is, however, only one arrangement at the BT plug end. Phew!).

Note that in the USA, the middle pair of the RJ11 (pins 3 and 4) carries the phone line. Therefore, equipment that has been brought from the USA will assuredly be like that, and will need a line cord that connects the RJ11 middle pair to BT pins 2 and 5. It does not follow, however, that equipment supplied in the UK by US companies will necessarily be like this. Several readers have UK versions of gear made by US companies, that definitely has the phone line on pins 2 and 5 of its RJ11 socket.

Specialist shops such as Maplin or Tandy should be able to supply either kind of RJ11-to-BT line cord.

A US traveller unable to get hold of a correct cord could, say, bring a US phone cord with them, buy a British-style extension cord (readily available here), cut both cords in half and splice the appropriate halves together.

does it matter if A and B are interchanged?

Interchanging A and B on the incoming line will merely reverse the polarity of the lines. Since normal telephone equipment is designed to work on either polarity, this would not matter. (But the installer should get it right, even so.)

(One contributor asserts that ALL phone equipment will work irrespective of polarity. Reference was made to the UK (BABT) approvals procedure, which lays down that equipment must be unaffected by polarity in order to be approved for UK use. Some informants seem to know of equipment that will only work on one polarity, although it was not claimed that this equipment had been approved for UK use: the best that can be said with confidence is that such gear would be unusual.)

Interchanging A and B between the master socket and a secondary socket, or on the line cord between socket and equipment, will prevent the bell from working (if the ringer is connected in the "British" fashion), and can cause other problems.

Why that 470 kohm resistor in the master socket?

This allows the exchange's tester or "routiner" to test the line, even if all the customer's telephones are unplugged.

Version: 2.10 Last-modified: Time-stamp: <96/05/12 23:04:34 jrg>