A voice telephone system feature that provides for an emergency mode of operation in the event of a commercial power failure. Also known as power failure cut-through, this feature allows an analog central office (CO) trunk to cut through the PBX switch to a predetermined analog emergency phone in a location such as the receptionist's desk, the security office, or the data center.There may be many such lines if a large PBX serves many users. Because analog CO lines and trunks are linepowered, i.e., powered from the CO, and the telco generally has several levels of emergency power backup in place, they are not highly susceptible to commercial power failures. This feature is particularly important if the user organization has no power backup such as an Uninterruptible Power Supply (UPS). Large organizations that consider telecommunications to be mission-critical generally have UPS systems in place that may include not only battery backup but also backup diesel power generators that serve to recharge the batteries during long outages. Many PBXs offer optional internal battery backup that is sufficient for one to eight hours of operation. See also UPS.