47 CFR § 95.377 - Tones and signals.
Personal Radio Service stations that transmit voice emissions may also transmit audible or subaudible tones or other signals for the purpose of selective calling and/or receiver squelch activation. These tones and signals are ancillary to voice communications and are considered to be included within the voice emission types, e.g., A3E, F3E, and G3E.
(a) Tones that are audible (having a frequency higher than 300 Hertz), must last no longer than 15 seconds at one time.
(b) Tones that are subaudible (having a frequency of 300 Hertz or less), may be transmitted continuously during a communication session.
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