The basic Central Control Unit (CCU) starts off as an 8 extension unit, with either analog POTS lines or ISDN line modules available. There is a serial connection that can be used for call logging and configuration. It can either use standard telephones or proprietary feature phones. A special Door Phone may be added on extension 23 and the system may be connected to an electronic lock relay to allow the door to be opened via a Door Phone call. Any extension may be programmed as a PA (Public Address) speaker.