A busy signal (or busy tone or engaged tone) in telephony is an audible call-progress tone or audible signal to the calling party that indicates failure to complete the requested connection of that particular telephone call. The busy signal has become less common in the past few decades due to the prevalence of call waiting and voicemail. An otherwise unspecified busy signal indicates that the called number is occupied. Countries have different signaling tones that act as busy signals, in most cases consisting of a tone with equal on/off periods at a rate of between 60 and 120 interruptions per minute.

Source: Wikipedia