In telephone systems, these tones are in-band indicators to subscribers, as opposed to in-band and out-of-band switching Signaling (telecommunications) tones. Typical major signal tones are dial tone, ringing tone, busy tone, and number unavailable tone. A loud annoying interrupted warning signal tone is used to alert subscribers of an engaged telephone circuit which will not ring, warning that the other party has disconnected as a reminder to indicate the handset is not properly hung up.
Subscribers may choose to include additional Vertical service code Custom Local Area Signaling Services such as Call Forwarding which may indicate function by a rapid period of interrupting the dial tone or tones of higher or lower pitch.
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