Teenagers discovered that the test facility could be used as so-called beep lines, in which they would dial up the primary number and wait for someone at random to call its mate.
[1] Phreaks would use the loop around in a similar manner, to exchange information that they had learned about the phone company.
[2] Loop lines are far less common today than they were in the 1960s, however they reportedly continued to exist past the start of the 21st century.
The most common protection techniques are: Eventually, telephone companies designed devices to prevent this misuse of the loop around.
Some devices blocked the audio if frequencies other than the milliwatt tone were present, which thwarted the average abuser, but skilled violators discovered that playing a 1000 Hz tone while speaking softly, the suppression unit could be subverted, and a notch filter could be used while listening to remove the tone at the other end.