47 CFR § 80.74 - Public coast station facilities for a telephony busy signal.
(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%.