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How a Telephone Line Works
Telephones are essentially very straightforward audio devices with a send path, receive path and a means of generating dialing. Any confusion usually lies with the way the device is interfaced with the line/ exchange or with the many features available on modern equipment.

This is a pair of wires running from the apparatus (telephone, answering machine, etc) to the local B.T.
exchange. The network of exchanges throughout the country is known as P.S.T.N. (Public Switched Telephone Network). The line has an impedance of 600 ohms and is terminated at the subscriber's end with a modular jack socket known as a master socket (NTE5). Additional sockets may be connected in parallel with the master. These should be secondary sockets which have no components inside.When a piece of equipment is approved it is given a REN (Ring Equivalence Number). This is a way of defining the current it takes from the ring signal. The REN of all equipments connected to a line must add up to 4 or less. So much for the line, now let's look at some of the voltages and see when we can expect to see them.

In this state no calls are in progress and there is no ring signal present. A nominal
50 volts DC can be measured on the line.
Now the exchange has superimposed the ring signal onto the 50v, In its most common format this is an AC waveform 75vRMS 25Hz with a cadence of 0.4 sec
on, 0.2 sec off, 0.4 sec on, 2 sec off.

The apparatus takes the call by putting a loop across
the line; this stops the ringing at the exchange and causes the 50 volts DC to fall to approximately 10 volts DC. Communication may now take place.

Other tones are generated at the exchange from time to time, e.g. dial tone, ringback tone. The parameters of these vary with line loss and type of exchange. However, they are not critical to the operation of most equipment.
Let's now take a look at some of the signals generated at the customer's end of the line. The most obvious of these is dialling and there are two types:

This feature is commonly referred to as Tone Dialling. Each key on the keypad selects a tone from a group of high frequencies and one from a group of low frequencies. The two tones are combined and sent to line (either for a preset time or for as long as the button is held down).

Whenever a key is pressed the loop formed when the apparatus went "off hook" is pulsed, 5 times for key 5, 10 times for key 0. In this country the pulsing or loop disconnection is done at 10 pulses per second. This is quite slow compared with tone dialling.
This function is used on PABXs to transfer an incoming call to another extension. There are two types of recall, Timed Break and Earth Loop. Timed   break   recall   causes   the   loop   to disconnected for approximately 75ms. With earth loop, the recall switch connects line (B) to earth (pin 4 of the modular socket).

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