A
busy signal is generated when you
dial a phone number that is currently in use. If the phone number is part of a rotary, a busy signal will sound if all the lines in the rotary group are in use.
A reorder signal sounds like a "fast" busy signal, but it is not a busy signal at all; at least not a signal telling you the number you are calling is busy. Rather, it means that a switch somewhere along the route of the call is at capacity and cannot complete your call. Just hang up and try again.
Reorders have been replaced by "We're sorry, all circuits are busy now..." in some areas. I wonder how long it'll be before busy signals will be replaced by a long and wordy message, like "We're sorry, but the number you are trying to reach is currently busy. Please hang up and try your call again later." Considering how stupid some people seem to be these days, that's probably not far off.