Signal tone

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.
Subscribers may choose to subscribe to additional services, such as call forwarding, which may indicate function by a stutter dial tone.