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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.
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.
A dial tone (dialling tone in the UK) is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is working and is ready to initiate a telephone call. The tone stops when the first dialed digit is recognized.
In the United Kingdom, the busy tone is a single 400 Hz tone with equal 0.375 s on/off periods. This was the case even when the UK was still part of the EU. The current 400 Hz/375ms 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 pulse duration, 0.75 s on, 0.75 s off.
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.
[1] [2] Typically, the disconnect tone is a few cycles of the reorder, busy, or the off-hook tone (e.g. in US), or between five and fifteen seconds of the Number Unobtainable tone (e.g. in UK). On some telephone exchanges in the UK, the following audio message is looped for fifteen seconds, interspersed with special information tones (SIT), to ...
Another solution would have been to use a separate set of "command pins" dedicated to sending and receiving commands; another could have used a signal pin indicating that the modem should interpret incoming data as a command. Both of these had hardware support in the RS-232 standard. However, many implementations of the RS-232 port on ...
The Do Not Disturb or (DND) function on most PBX or PABX systems prevents calls from ringing on an extension for which DND is activated. Some Do Not Disturb (DND) attributes include directing the call to a preassigned extension (like a secretary or assistant), busy signal, DND signal, or recorded message generated by the telephone switch.