Which device allows multiple telephone extensions to access a group of single telephone lines?

A key system is a type of telephone system designed to allow multiple telephone extensions to share a limited number of phone lines. This system enables users to access and control those lines easily, allowing for efficient communication within an organization. Each extension can typically have its own line, but in the case that all lines are in use, users can share the other available lines among the extensions.

Key systems often include features such as intercom functionality, hold and transfer capabilities, and lights that indicate line status, making it convenient for users to manage incoming and outgoing calls in a busy environment. This operational structure is particularly beneficial for small to medium-sized businesses that require multiple extensions but operate with a limited number of incoming lines.

While other devices like Centrex provide similar functionalities for managing calls, they generally rely on service providers to deliver services rather than being on-premises equipment like key systems. Trunks and FX lines pertain to the backbone or incoming/outgoing communications in a network but do not serve the same purpose of connecting multiple extensions directly to a set of lines as a key system does.