busy signal definition by American Heritage Dictionary
busy signal - Computer Definition
An audible (or visual) signal indicating to the calling party that the called circuit is engaged and, therefore, unavailable. In the public switched telephone network (PSTN), a central office (CO) sends the busy signal in the backward direction, i.e., back to the caller. A fast busy signal, or reorder tone, is an indication to the calling party that network resources are not available to process the call. See also PSTN.