Hacking Question:

A loop is two phone numbers connected together by the phone company
for testing purposes. A loop has a high end and a low end. If you
dial the high end, you will hear nothing. Not even a ring. If you
dial the low end, you will hear an annoying 1,000hz tone for several
seconds. If you connect to the high end and someone dials the low
end, you can speak to each other.