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 handsets.
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 feature access.
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.