Listen to a busy signal from North America.
Listen to a busy signal from North America used prior to 1980.
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A busy signal (or busy tone or engaged tone) in telephony is an audible or visual signal to the calling party that indicates failure to complete the requested connection of that particular telephone call.
There are several distinctly different types of busy signals:
- a reorder tone, (sometimes called a fast busy signal), indicates that no transmission path to the called number is available. It is often played after a recording describing a telephone problem;
- an otherwise unspecified busy signal indicates that the called number is occupied, if that number is calling out, if someone else has called the number or is calling the number at the same time, if the other line was left off-hook or it is otherwise unavailable;
- this tone sometimes occurs (sometimes with an intercept message playing over the busy signal) at the end of a call to indicate the other party has hung up. See disconnect supervision.
Countries have different signaling tones that act as "busy signals", in most cases consisting of a tone with equal on/off periods at a rate of between 60 and 120 interruptions per minute.
In North America, the Precise Tone Plan used today employs two tones of 480 and 620 Hz at 60 i.p.m. (i.e. on for 0.5 sec., off for 0.5 sec.). In the past, before the adoption of the PreciseTone system, busy signal was generally composed of the same tone as dial tone in the central office in question, interrupted at the same rate.
In the United Kingdom, busy tone consists of a single 400 Hz tone with equal 0.375-sec. on/off periods. This tone was adopted in the mid to late 1960's and replaced the older busy tone which was the same 400 Hz signal but at half the interruption rate (i.e. 0.75 sec. on, 0.75 sec. off).