"A signal tone or signalling tone (or signaling tone) is a steady or pulsating periodic sound (not necessarily a pure tone) used to indicate a condition, for example on a telephone line or as an audible warning.
In telephone systems, signaling tones are used as call progress tones for in-band indications to subscribers. Certain telephone switching systems used tones, in-band or out-of-band, for signaling on trunks.
Typical well-known call progress tones are dial tone, ringing tone, busy tone, and the reorder tone. A loud stutter tone is used to alert subscribers of a handset left off-hook, effectively disabling the circuit for receiving calls." - (en.wikipedia.org 31.07.2021)
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