A proprietary telephone is an extension custom designed to
operate with the associated system. They are not usually able
to operate in conjunction with any other system.
Systems that operate only with its own proprietary telephones
are becoming less common and most modern systems are hybrids,
allowing the connection of proprietary and standard SLT type
The advantage of fitting proprietary telephones is that they
will usually offer a closer integration with the system and
its features than a SLT. These extensions will often have
illuminating keys representing the lines and other extensions
on the system and may have other custom keys allowing immediate
The proprietary handset is often more expensive than the SLT
option. However the operation of the SLT is more convoluted
when accessing the more complex system features and in some
cases there are features which the SLT cannot access.
Where the system is equipped with handsets, which have illuminated
line keys for selection and control of lines the system is
referred to as a 'keysystem'. If the system allows only for
standard SLT extensions it is often referred to as a 'PBX"
even though this is not a true use of the term. If it allows
a mixture of proprietary and SLT handsets it is referred to
as a 'hybrid' system i.e. one which can operate as part keysystem
and part PBX as they are defined here.