"For some time the New York Police Department has been employing the municipal broadcasting station WNYC to broadcast descriptions of missing persons and other features of police work in which it is desired to enlist the cooperation of the public. The success of this program has been such that the Police Department wished to equip the precinct houses and police booths located  in various parts of the city with receiving sets with which they could listen in on communications from the headquarters station WNYC. The fundamental requirement was signaling apparatus incorporated in the receiving sets which would attract the attention of the attendant at the proper times. The system which was finally developed by the engineers of the Bell Telephone Laboratories, Inc., in cooperation with the New York Police Department is an excellent illustration of the adaptability of wire practices to the radio field..."

Source: Anderson, S.E., "Radio Signaling System for the New York Police Department," The Bell System Technical Journal, October, 1926, pg. 529.