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.
The tone characteristics vary by country or telephone administrations. In North America it is a dual-frequency tone of 620 Hz and 480 Hz interrupted 120 times per minute. It is roughly a slightly low D-sharp over a noticeably low B-natural on an equal-tempered A440 scale, i.e., an interval very close to an equal-tempered major third and fairly close to but noticeably wide of a just major third) at a cadence of 0.25 seconds on, 0.25 off, i.e., two beeps per second. The British reorder tone is the same 400 Hz tone as the busy tone, instead with a cadence of 0.4 seconds on, 0.35 seconds off, 0.225 seconds on, 0.525 seconds off.
The signal is used to indicate that the call is unreachable, either because all circuits (trunks) are busy, the called number is out of service, the call is unroutable, or sometimes that an invalid code has been dialed. A PBX also often indicates an invalid extension, while dialing an invalid telephone number on the PSTN usually results in playing the triple special information tone and a recorded announcement. Many telephone companies in North America and in the United Kingdom often play the reorder tone after the reading of a recorded announcement explaining the reason of call failure, typically when the other party fails to receive the call after ringing for several minutes. The reorder tone is sometimes confused with the busy signal because they use the same frequencies and differ only in that the busy signal's beeping sounds have a slower cadence than those of the reorder tone.