Connecting People

Key System Unit


Key System Unit

The Key System Unit (key telephone system or key phone system) is a multiline phone system that is efficient for up to 50 users. A KSU unit relies on the user to select an external line, rather than the automated exchange of Private Branch Exchange (PBX) phone systems.

The term ‘key’ derives from an original Bell System term for a manually operated switch, such as the line-button on early telephones. ModernKey System Unit phone systems have a central hub (the key system unit) that provides functionality, such as internal calling, that non-key phone systems can’t offer.

A variation of the Key System Unit phone system is the KSU-less system. This offers generally similar functionality but without some of the more specialised (and expensive) technology of the pure KSU system. Cheap and simple to set-up, KSU-less systems are generally only viable for up to 10 users.

Nowadays, the distinction between he largestKey System Unit and corporate-level PABX systems is increasingly blurred, with the main differentiators being the quantity, scope and sophistication of available functionality. Modern hybrid key systems are virtually small PBXs, fully digital systems that, in some cases, even embrace the latest VoIP technology.


  • Efficient for up to 50 employees
  • Convenience of proprietary handsets
  • Latest hybrids combine key system and digital functionality
  • Proven one-size-fits-all functionality
  • Easy customisation through software upgrades
  • Reliability and ease of maintenance



  • Possible limitations for larger businesses
  • May lack some functionality of full-blown PBXs

Contact us today for a free evaluation and business quote. Lantel is the number one business phone systems business in Ireland.