The "cuckoo tone", named after its distinctive pattern of alternating high and low tones, is the sound emitted by all BT payphones on answering an incoming call, and also when calling the operator from a payphone. Its purpose is to alert the operator to the fact that the line in question does not, or should not, accept incoming calls.
Phone phreakers found ways to disable the cuckoo tone, meaning that reverse-charge or collect calls could be made from any line to a public telephone. Usually the presence of the tone allowed the operator to prevent such calls being made. However, though every operator in the UK is trained to recognise the cuckoo tone and its significance, certain international operators may think nothing of it and connect the call regardless.
The tone itself can be removed from the payphone at the customer's request for a service charge of around £10, should this ever be required.