Ringdown

Ringdown: In telephony, a method of signaling an operator in which telephone ringing current is sent over the line to operate a lamp and cause the drop of a self-locking relay. Ringdown (a) is used in manual operation, as distinguished from dialing, (b) uses a continuous or pulsed ac signal transmitted over the line, and (c) may be used with or without a switchboard. The term "ringdown" originated in magneto telephone signaling in which cranking the magneto in a telephone set would not only "ring" its bell but also cause a lever to fall "down" at the central office switchboard.

Source: from Federal Standard 1037C and from MIL-STD-188