Section 80.74 - Public coast station facilities for a telephony busy signalA "busy" signal, when used by a public coast station in accordance with the provisions of § 80.111(d) , must consist of the transmission of a single audio frequency regularly interrupted, as follows:
(a)Audio frequency. Not less than 100 nor more than 1100 Hertz, provided the frequency used for this purpose will not cause auto alarms or selective-ringing devices to be operated.(b)Rate of interruption. 60 times per minute ±10%.(c)Duration of each interruption. 0.5 second ±10%.