All equipment which is designed for use on a single analogue
exchange line is designated as a simple telephone. These may
variously be referred to as Simple Telephones, Single Line
Telephones (SLTs), Plain Ordinary Telephones (POTs) or if
connected to an inherently digital system Analogue Telephones.
All these names refer to the same class of equipment.
This type of equipment includes the likes of cordless telephones,
answering machines, facsimile and modems. They all work on
the same basic principles with certain characteristics enhanced
to perform specific functions.
The basics of a simple telephone device are shown in Figure
The Basic Units Of A Simple Telephone
In Figure 4 the essential components of a simple telephone
are separated to show their relation to each other and allow
easier discussion. The handset usually contains the Microphone
(M) and Earpiece speaker (S). The Base section contains the
Hook switch (H), Ringer detection and signalling (R) and Dialling
circuits (D). Connection to the line is via a single pair
The functions of microphone and earpiece require no further
explanation when referring to a speech device. In a data device
they would be replaced by transmit and receive circuits. In
an answering machine the microphone is usually absent and
the earpiece feed directly to the answering circuit.
Power is fed from the line to operate these basic modules.
However more sophisticated equipment will have additional
power supplies to power their more complex functions.
The hook switch turns the telephone on and off. It is often
a spring loaded micro switch held in the open position by
the handset resting on a lever connected to its contacts on
the most basic type of simple telephone. Automatic equipment
uses and electronic switch which may be controlled via the
handset being on the base or the logic designed into the telephone,
E.g. Answering machine or cordless telephone. Problems can
arise from electromechanical switch types as the contacts
wear with use and send spurious signal to line when the switch
does not open and close cleanly. This will manifest itself
in the form of ringback caused by jitter or may disable the
telephone altogether by not breaking and leaving a permanent
busy state to the line. Devices, which use a solid state electronic
hook switch, do not suffer from these problems since there
is no component wear to the switch device.
The ringer module will detect an incoming ring signal and
trigger a sounder or bell to alert the user that a call is
being received. This will only operate with the hook switch
in the open position i.e. the telephone is in the idle state.
The ringer will consume current from the line and the level
of this current is used to derive the Ringing Equivalence
Number (REN) of the device. A domestic telephone line can
normally support a REN of 4. Most modern equipment will have
a REN of 1, so upto 4 can be connected before the REN is exceeded.
Should the particular telephone have an external power supply,
it may require less than 1 REN of power, yet be rated as REN1
since this is the lowest rating on the REN scale. In such
cases it can sometimes be possible to connect more than 4
devices, providing the total does not exceed REN 4. If the
REN is exceeded ringing may become erratic or stop altogether.
Care should be taken when connecting older equipment as these
units can often have a REN of 3 or more.
The last essential element is the dialling circuit. This creates
and sends the dialled digits to the line in response to the
dial pad or rotary dial wheel. Modern equipment will allow
switching between pulse and tone dialling modes so that the
dialling type can be matched to the line. It is unusual to
require pulse dialling today. In an answering machine the
dialling circuit is absent since there is no requirement to
These elements are found to a greater or lesser extent in
all simple telephone equipment. However the design of the
electronics often makes differentiation between the functions
impossible for anyone but trained technical staff.