A colleague just gave me this juicy tidbit. He plugged a phone into the jack and the phone immediately rang.
Phone immediately rang again.
This repeated 24 hours a day until the Army put a trace on the call. The trace led to, get this, a Coca-Cola machine. The manufacturer had built into these vending machines the capability to call the bottling company when they were getting low on supplies and order more. The Army cut the telephone cable on the Coke machine.