Telephones are essentially very straightforward audio devices with a send path, receive path and a means of generating dialling along a copper cable. Any confusion usually lies with the way the device is interfaced with the line/ exchange or with the many features available on modern equipment.
The Line
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.
Idle line state
When the telephone line is in idle state no calls are in progress and there is no ring signal present. A nominal 50 volts DC can be measured across the phone line.
Ringing line state
When the telephone line is ringing on an incoming call the exchange has superimposed a ring signal onto the 50V. In its most common format this is an AC waveform 75V RMS 25Hz with a cadence of 0.4 sec on, 0.2 sec off, 0.4 sec on, 2 sec off.
Line in use state
The telephone 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.
The Apparatus
Tone Dialling
This feature is called Dual Tone Multi-Frequency (DTMF) dialling, but more 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 pre-set time or for as long as the button is held down.
Pulse Dialling
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.
Recall function
This function is used either when using Star Services on the BT network or 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).