A signal tone or signalling tone (or signaling tone) is a steady periodic

Periodic function

In mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...

 sound used to indicate a condition, for example on a telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 line or as an audible warning.

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

Dial tone

A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...

, ringing tone, busy tone, and number unavailable tone

Reorder tone

The reorder tone, also known as the fast busy tone, is the congestion tone or all trunks busy tone of a public switched telephone network. It varies from country to country; in the USA it is a dual-frequency tone of 480 Hz and 620 Hz at a cadence of 0.25 seconds on, 0.25 off; that is two beeps per...

. 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

Vertical service code

A vertical service code or VSC is a special code dialed prior to a telephone number that engages some type of special telephone service...

 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.

See also

• Musical tone
  Musical tone
  A musical tone is a steady periodic sound. A musical tone is characterized by its duration, pitch, intensity , and timbre . The notes used in music can be more complex than musical tones, as they may include aperiodic aspects, such as attack transients, vibrato, and envelope modulation.A simple...
  
  
• Dual-tone multi-frequency signaling
• Signaling (telecommunications)