busy signal Hear it!

busy signal Telecom 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.