Busy line interrupt

Busy line interrupt is a function on land line telephones that allows a caller to interrupt a phone conversation of another caller, especially if the caller does not have call waiting.

Much like long distance calling before the late 90s, busy line interrupt is a service provided in order to, for instance, 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 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 emergencys as noted above.