In a public switched telephone network, busy verification is a network-provided service feature that permits an attendant to verify the busy or idle state of station lines and to break into the conversation.
A 440 Hz tone is applied to the line for 2 seconds, followed by a 0.5 second burst every 10 seconds, to alert both parties that the attendant is connected to the circuit.
References
Busy verification Wikipedia(Text) CC BY-SA