A fast busy signal, sometimes called a reorder tone, indicates that there is no way to reach the number dialed. Reorder tones are most often played following a recorded message describing the problem encountered with an attempted call.
There are many problems and issues that may interrupt service or leave callers unable to reach the number they have dialed. According to the Telecommunications Industry Association, a reorder tone is used when there is no valid transmission path, meaning that the number dialed is unavailable. A reorder tone is very similar to the busy signal used to indicate a line that is in use, but it is noticeably faster.