A telephone system is an integral part of business today.
All but the smallest businesses have their own private system.
Even some homes now have them. These systems are simply smaller,
private, exchanges, which allow the subscriber lines to be
shared between the connected telephones called extensions.
The private telephone system allows the owner more control
over the extensions connected, provides more features and
functionality than the subscriber lines alone and allows a
large number of users access to the telephone without having
to provide a line for each. The first systems were installed
in the early part of this century and have been popular ever
since with business users and large houses.
A telephone system is much the same as the network exchange
to which it is connected. The exchanges allow the relatively
large number of subscribers to have access to the comparatively
small number of lines. Since they do not usually all make
their calls at the same time they each perceive that they
have their own private line apparently always available. This
is the basic concept behind the systems installed by the private
individuals or companies using the network .
They are variously known as Private Automatic Branch Exchanges
(PABX), Private Branch Exchanges (PBX) or Call Routing Apparatus
(CRA) and all these terms are interchangeable in today's marketplace.
Besides allowing the sharing of lines by the extensions they
offer increasingly sophisticated additional functions which
the network cannot. Even the most basic systems offer call
hold, transfer and inter-extension calling besides just making
and receiving calls. Larger and more complex systems provide
wider ranges of functions from call forwarding, music on hold
and short code dialling of centrally stored telephone numbers
to automatic route selection, multi-site operation and selective
call barring options.
The features will depend upon the manufacturers response to
market demands and are too numerous and fast changing to be
described in detail here.