CO lines An ESI Communications Server can operate either on a station-by-station basis as a PBX or as a combined key/PBX using standard loop-start lines. If a station has line keys programmed, the user accesses the lines by pressing one of these keys or by dialing the line group number 9 (or 8 or 71–76). If a station does not have line keys programmed, the user always accesses CO lines by dialing 9 (or 8 or 71–76). Since the system handles call transfer and auto attendant functions efficiently, operating in the PBX mode provides more programmable feature keys for other uses and the opportunity for glare is greatly reduced. Notes: When a port card is added to or removed from the system — i.e., thus changing the number and configuration of cards in the system — you must reprogram the CO lines. However, if a port card is replaced by the same type of port card (e.g., when you replace a faulty 684 card with a new 684 card1), you don’t have to reprogram the CO lines. As a visual indication of CO line usage, the phone’s display will show on/off-hook line status. All phone programmable keys default to being unprogrammed (except on extension 100, where the first key defaults as a day/night key). Use extension button mapping (Function 35; see page G.36) to assign line keys system-wide. An individual station’s keys can be reassigned using either PROGRAM 2 or “radio-key programming” at that station.