Busy signal

A busy signal (or busy tone or engaged tone) in telephony is an audible call-progress tone or audible signal to the calling party that indicates failure to complete the requested connection of that particular telephone call.
The busy signal has become less common in the past few decades due to the prevalence of call waiting and voicemail.
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.
The Beep-Beep Line or Jam Line.
In the early 1960s through the early 1980s, a telephone busy signal provided an early form of social media in many cities and towns of the United States called the "Jam Line" or "Beep Line". Due to a flaw in the telephone switching equipment, teenagers discovered they could talk to each other over the busy signal, often exchanging phone numbers, mostly for the purpose of dating. Common phone numbers for this to form were on popular radio station request lines, where teens would be calling in en masse to try to win concert tickets or request their favorite songs, thus "jamming the lines" and generating a perpetual busy signal.
Reorder tone.
A reorder tone, sometimes called a "fast busy" signal, indicates that no transmission path to the called number is available. This 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 Europe, the ETSI recommendation for busy tone is a single 425  Hz tone 60 i.p.m. (i.e. on for 0.5 s, off for 0.5 s.)
This is used in all EU countries, and many others following ETSI standards. 
It is also found on mobile phones that follow the GSM & 3GPP family of standards and may be encountered on ISDN equipment and PBX/office systems found outside Europe, which may follow ETSI standards.
ETSI tones are also used in Ireland, even though the ringback tone used is the same as the UK.
Some French networks continue use a 440 Hz tone in place of the standard 425 Hz tone, but this is gradually being replaced.
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).