Also known as call return. A network-based CLASS service of the public switched telephone network (PSTN).When activated by the caller who reaches a busy line, the central office (CO) monitors the target telephone number for a period of time, e.g., 30 minutes, and advises the caller with a (usually distinctive) callback ring when that line becomes available.When the caller answers the ringback call, the CO automatically redials the target number. See also CLASS and PSTN.