For most of the 20th century, calls were usually placed on the
public switched telephone network via electromechanical switching equipment. When a caller dialed a number that was busy or permanently unavailable, the
central office of their carrier would shunt the incoming call to a circuit on which the
busy signal tone was produced. These busy signal circuits did not have their voice path cut off, and as a result, if two or more people reached the same busy signal, they could potentially talk to each other and host a conversation over the sound of the busy signal. The majority of participants were teenagers using these lines to hold informal conversations with strangers in their locality, as well as to collect the phone numbers of potential dates and friends. Beep lines were also a popular spot for
phone phreaks, or people who deliberately experimented with and explored public telephone networks, during the 1970s. As central offices did not send
answer supervision to busy signals, conversations hosted over these so-called "beep lines" were
toll-free in most cases. Another entry point was permanently unavailable numbers or
loop-around test numbers intended for internal use by the telephone company. While beep lines were not initially illegal in the United States, Some measures within central offices to quash beep lines included making the busy tone louder or by increasing the interruptions per minute of the tone, to the chagrin of regular callers who found these new tones obnoxious. or by upgrading the central office equipment to
electronic switching systems (ESS). as beep line enthusiasts would cycle through secretive beep lines or connect to distant exchanges with older electromechanical switching equipment and talk there. ==See also==