Ringdown
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In telephony, ringdown is a method of signaling an operator in which telephone ringing current is sent over the line to operate a lamp or cause the operation of a self-locking relay known as a drop.
Hotline telephone without dial
Business telephone system
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A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system to the private branch exchange.
Ringdown
…is, going off-hook at one end (end A) rings the other (end B). Going off-hook at end B has no effect at end A. Ringdown features are often part of a key telephone system. In the wire spring relay key service units of the Bell System 1A2, a model 216 automatic ringdown was used to operate the circuit. In the 400-series…
Early-1970s Telephone Exchange model TESLA Automatic system
Early model 464G Western Electric key telephone set
A 1930s key telephone for a private branch exchange
Nortel T Series Key System Telephone