The engaged tone is an intermittent audio signal sent back to a caller when the destination line is already active or when switching equipment lacks capacity. In traditional telephony, it consists of a repeating cadence of tones, typically 400 or 480 hertz, interrupted at regular intervals. Telecom engineers measure its cadence to troubleshoot routing failures or trunk congestion.
Heard on the job
I tried looping back to the test number but just got an engaged tone on the second trunk.
Insider tip
In British and Commonwealth engineering, this is the standard term, whereas North American technicians almost exclusively say busy signal.
Also heard as
- busy signal
- busy tone
Do not mix it up with
- Fast busy signal: An engaged tone means the end user is on the phone, while a fast busy signal indicates network congestion or a routing failure.