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.
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The busy signal has become less common in the past decade due to the prevalence of call waiting and voice mail.
Reasons for a busy signal
An otherwise unspecified busy signal indicates that the called number is occupied:
The standard busy signal sometimes occurs (sometimes with an intercept message played over the busy) at the end of a call to indicate the other party has hung up (see Disconnect tone), but mostly the Off-hook tone is used. In some phone companies in the United Kingdom, the busy signal is played after the dial tone to indicate the caller has used up their allocated time to dial a number and must hang up, before the off-hook tone is played.
Reorder tone
A reorder tone, sometimes called a fast busy signal, indicates that no transmission path to the called number is available. This is can occur either because the Inter-LATA trunk is busy at the time of the call- in which case this clears in a few seconds, if one redials- or the number is temporarily out of service, due to maintenance or the number is not willing to accept calls. It is otherwise played after a recorded announcement explaining the reason for a general call failure.
Styles
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 (i.p.m.).
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 s, off for 0.5 s). 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 s on/off periods. This tone was adopted in the mid to late 1960s and replaced the older busy tone, which was the same 400 Hz signal but at half the interruption rate (i.e. 0.75 s on, 0.75 s off).