Busy line interrupt
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Busy line interrupt is a function on land line telephones that allows a caller to interrupt a phone conversation of another caller,[1] especially one who does not have call waiting.
Much like long distance calling before the late 90s, busy line interrupt is a service provided, for instance, to allow people to contact a family member in the event of an emergency if the receiving party's line is busy and the news cannot wait.
The service is not free. It is usually about $3 per use[2] and is charged whether or not the calling party can reach the person on the other end (as the phone just might be off the hook for privacy reasons or the line might be connected to the internet). For this reason, it is usually only used in cases of emergency's as noted above.
In the early 1990's, reports were surfacing that "phone phreakers" were having success in manipulating the line interruption function with signaling tones and equipment that emulated the telephone operator's. It was claimed to be possible to eavesdrop on any engaged telephone line by number.
