Private Branch eXchange (PBX) systems are for medium to large size companies. They come in a variety of sizes and offer the most feature rich systems on the market. Larger systems are expandable to accommodate future growth.
Key System Unit (KSU)
KSUs are for small to medium sized organizations. They do not pack all the features of a PBX, but they can save money.
These are for offices of less than 10 employees that provide basic phone service and a minimal amount of features. There is no central base unit. All the functionality is contained in the individual phone sets. Features are extremely limited.