# Busy Tone

The central office asserts the busy tone after the number has been dialed, and if it is impossible to connect to the desired phone number. There can be several reasons one may receive a busy tone. The most common reasons are; the phone is already in use or there is congestion in the phone network. The standard busy signal in North America consists of a tone, or combination of tones, turned on and off at a 50% duty rate – ½ second on, ½ second off. Other busy tone cadences may be used for different situations that cause the dialed number to become unreachable.