When engaged in a conversation with a client, an agent may lose track of queue conditions. A Call-Waiting Cue is a short tone, only audible to the agent, played with certain queue conditions - long holding times or large amount of inbound calls.
The tone alerts the agent that he should pick up the pace in order to provide a general good customer service for all callers in line. This is done discreetly and without any involvement of a manger, allowing the agent to remain professional and estimate the quickest resolution to the current call.