Emergency phone breakthroughs and busy line interrupts can help you get through when the the person you need to reach is already on the phone and does not have call waiting. Both can be done anytime that you must reach the person on the other end of the phone, but in the case of an emergency breakthrough, you will also be able to find out if the phone is out of service.
Make Sure the Line is Busy.
Call the number multiple times and ensure that the line is giving you a busy signal. A "fast busy" may mean that the line is out of service, as opposed to just being used. If the number is busy, and you are worried about the person on the other end, or have an urgent need to speak to them, you can try to do an emergency breakthrough.
Call the Operator.
Dial "0" for the operator. When you get one on the line, tell him that you have been attempting to reach a number, but it is busy. Request that they perform an emergency breakthrough or a busy line interrupt. The operator will tell you the cost for the service. If you confirm that you still want the breakthrough, the operator will break into the phone call and let the person know that you are trying to reach them.
Call the Number Again.
The operator may tell you whether the person accepted the breakthrough or not. If it turns out that there is a problem with the line, the operator will let you know. You will need to call the person back and see if you can get through. The person on the other end does not have to accept the breakthrough and can simply ignore your call.
Law enforcement may, at no cost, request an emergency breakthrough on any line, if they feel there is potential danger, like in a hostage situation.