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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.
The reorder tone, also known as the fast busy tone, or the congestion tone, or all trunks busy (ATB) tone is an audible call progress tone in the public switched telephone network (PSTN) that is returned to a calling party to indicate that the call cannot be processed through the network.
In many cases, when calling from abroad, busy, reorder and other call failure tones may be played by the local switch. Modern signalling protocols like SS7 send this information digitally; thus only a ringback tone or announcement generated by a distant switch in a foreign network will ever be heard by callers from other countries or networks.
Disconnect supervision provides a signal that the call has been disconnected. For example, the called party may indicate to the telephone exchange that the call is being disconnected by the called party by allowing loop current to flow in the line, or the called party indicates to the exchange that the call is being answered.
Busy override is a function of the private branch exchange that allows the calling party to override the busy signal on the called party to break into the ongoing conversation. Before breaking in most PBX announce the incoming call by a distinctive sound signal or tone , but in most cases this can be disabled by software.
In the past, the call would be forwarded to an intercept operator after usually two readings of the message; today, however, this procedure is not observed, and on some systems a fast busy signal follows the second reading of the message instead. (A busy or an Off-Hook may be used depending on the provider.)
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For most of the 20th century, calls were usually placed on the public switched telephone network via electromechanical switching equipment. When a caller dialed a number that was busy or permanently unavailable, the central office of their carrier would shunt the incoming call to a circuit on which the busy signal tone was produced. These busy ...