# Ringback Tone

After the telecom equipment has dialed the phone number, if the phone number is valid and not busy or blocked, the central office asserts an audible ringback tone. The ringback tone has a specific ON/OFF and repetition cadence – which in North America is 2 seconds on, 4 seconds off, 2 seconds on, 4 seconds off, and so on. Please note that the term "ringback" in this context is distinct from the same term that is sometimes used in describing sequences in annunciator systems.