A bit of technical information

This information is by no means complete, and indeed, it is still dynamic, and changing with use. For example, the once ubiquitous bell wire on British telephones is beginning to die out, along with the Loop Disconnect 'phones it served. The reasons are twofold. One, it overcomplicates the wiring from a DIY or non-technical point of view, but secondly, and most importantly, it can badly affect the performance of ADSL modems attached to that line.

UK Wiring

The following information applies to wiring telephones within the United Kingdom:

Cable Colour Cord Colour Pin Number Function
Green/White Orange 1 None
Blue/White Red 2 B wire (+ve)
Orange/White Blue 3 Bell Wire
White/Orange Green 4 PABX Earth
White/Blue White 5 A wire (-ve)
White/Green Black 6 None

US/Canadian Wiring

The following information applies to wiring telephones within the United States and Canada:

Cable Colour Cord Colour Function
Orange/White or Yellow Yellow Tip [Line 2] (+ve)
Blue/White or Green Green Tip [Line 1] (+ve)
White/Blue or Red Red Ring [Line 1] (-ve)
White/Orange or Black Black Ring [Line 2] (-ve)

For more information on wiring, and pinouts of various telephone plugs, see the Telephone connectors page.

Signaling Information

Loop Disconnect Dialling (Pulse Dialling)

  This method of dialling originated with the first automatic systems in the 1890's (an automatic system is one where the intervention of an operator is not needed for at least some calls, a system that is mixed, as in an operator required for long distance but not local calls, is referred to properly as an automanual system). This made greater privacy of calls a reality, but originally, many engineers were opposed to automatic systems as they left the technical task of setting up a call to the untrained subscriber, rather than a trained operator, however, eventually this system came to dominate telephone communications.

The basic methods of automatic telephony developed fairly quickly and were modelled after the central (or common) battery [CB] manual switching system, where the power to operate the telephone is transferred over a pair of wires from the exchange [central office]. This "local loop" can also be used for signalling the number required, as well as on and off hook conditions. The original signalling system that was almost universally used was Loop Disconnect signalling, where the loop current is interrupted a number of times depending on the number required... Once for 1, Twice for 2.... through to Ten times for 0.* The open circuit loop voltage can vary enormously from approximately 10 volts on some PABX (Private Automatic Branch Exchange) lines, to 100 volts on some long circuits in certain countries, however the principle remains the same. In telephony, the positive (+) side of the battery is usually earthed/grounded, thus making the voltage negative with respect to ground. In almost all systems also, the ringing is high voltage alternating current (usually 70 to 150 volts AC at 10 to 60 Hz {cycles per second}).

* A few countries such as New Zealand used what is known as "revertive impulsing", which means the number of pulses dialled is the difference between 10 and the number you dialled, which simply means the numbers are laid out on the telephone dial in the order 9,8,7,6,5,4,3,2,1,0 instead of 1,2,3,4,5,6,7,8,9,0. Mechanically the systems are the same, but electrically the order of the multiple wiring is reversed.

  Automatic telephone systems are more or less the same in most countries. The biggest difference is between North America and the rest of the world and isnot immediately obvious from even a detailed examination of the equipment. This is the fact that the ratios of make to break on loop disconnect (pulse) dialling are different. In North America, the pulse ratio is 40% make to 60% break. In most of the rest of the world, the ratio is 33% make to 67% break. The overall timing is 100 milliseconds (ms) per impulse, thus making the make break times as follows: 40ms make, 60ms break (US and Canada) and 33ms make, 67ms break (rest of the world).

Figure 1, Comparison of impulse ratios

Diagram showing pulse timing on a loop disconnect line circuit in the US and UK

I(l) is the loop current, point X is where
the telephone is taken off hook.
In both cases, the digit 3 was dialled.

  Another factor is the interdigit pause, which is required to allow the switching system to differentiate between pulse trains. This is nominally 400ms minimum although some systems can operate with an interdigit pause of less than 200ms.

Dual Tone Multi-Frequency Dialling (DTMF or TouchTone™)

  This is a voice frequency signalling system developed by Bell Laboratories in the United States around the mid 1960's. It was conceived originally to allow subscribers to dial directly over non physical lines, (for example carrier lines, or radio links). Since that time, it has become increasingly used due to the fast detection time (50ms) compared to Loop Disconnect dialling. A 10 digit number takes a mere second to autodial from a memory with DTMF, whereas if that number contains a large number of zeros, ( 555-000-0000 for example) it would take 14 or 15 seconds using Loop Disconnect. As switching systems have become faster, DTMF has become more popular as it allows full advantage to be made of the increased speed, whereas Loop Disconnect would still take just as long today as it did in 1900.... in fact longer, due to longer telephone numbers being in use in most parts of the world. There is a motion to phase out loop disconnect dialling by the telephone companies in most parts of the world, as the idle time during which a call is being set up costs them a small amount of money without being profitable. Less cynically, DTMF does cause less interference to adjacent lines than Loop Disconnect.

Figure 2, DTMF (TouchTone™) Frequencies

1209Hz 1336Hz 1477Hz 1633Hz
697Hz 1 2 3 A
770Hz 4 5 6 B
852Hz 7 8 9 C
941Hz * 0 # D


  1. The origin of the A, B, C, and D keys is in the US Military "Autovon" system, where they were used to assign priority to calls.
  2. the # symbol is now correctly known as an "octothorpe". This word was coined as a joke but it has stuck, as it is much less ambiguous than calling it a "pound sign" (which it is not, £ is a pound sign), or the "gate" symbol, as BT used to.

**TouchTone is a trademark of British Telecommunications in the United Kingdom, and of A.T & T Telecommunications in the United States.

Recall or Flash Signalling

  This is the third method of signalling the exchange equipment from a subscriber's premises equipment, and was originally used only on office telephone systems. Now with the advent of call waiting and similar services, it is required by most customers. There are 3 different methods of signalling this condition (used to put calls on hold, switch to a second call etc.)... The first is the method used by the Post Office Telecommunications Department in the UK, which is Earth (ground) Loop Recall, where the line pair is earthed (grounded) for a moment. Most telephones in the UK still have a ER or P-E switch (also marked LD-E).. the reason for this is that Earth signalling is compatible with Loop Disconnect and DTMF signalling systems, but the next method is only suitable for DTMF signalling. This next method is TBR (Timed Break Recall) which is a precisely timed 100ms break in the line current (one half longer again than a 1 dialled on a Loop Disconnect telephone). This feature is sometimes known as "Flash" although that is incorrect. Flash is the method of signalling the same condition in North America, which is a much longer break in line current (500 to 1000ms). This condition if applied in the UK would cause the line to be released, it all depends how the telephone system was built/programmed. The advantage of "Flash" is it does not require a dedicated button on the telephone, you simply "Flash the cradle switch" in the same way you used to re-call a long distance operator. (Hence the feature being called "Recall" in the UK).

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