The switch sends this repeating cadence of tones to the calling party when the destination line is already off-hook or when no trunk circuits are available to route the call. In North America, the standard cadence is alternating half seconds of tone and silence. If the network itself is overloaded rather than the end user, the switch generates a faster version to indicate a trunk blockage.
I tried testing the fax line but I just get a busy signal, so someone might have left the handset off the cradle.
Also heard as
- engaged tone
Do not mix it up with
- Fast busy signal: A standard busy signal means the end user is on the phone, while a fast busy signal means the network itself is congested or broken.
Related terms
Fast busy signal
A rapid pulsing tone indicating that the telephone network cannot route the call.
Engaged tone
An audible network signal indicating that the dialed number or circuit is in use.
Glare
A call collision that happens when an incoming and outgoing call seize the same trunk circuit simultaneously.
Blocked
A state where a call or connection attempt is rejected by the network switch or firewall.