黑箱
A call originating from a telephone fitted with a black box would still be charged for by the telephone company unless some method to circumvent the call charging was deployed. Black boxes were commonly built by phone phreaks during the 1960s to 1980s (and in some places like Eastern Europe, well into the 2000s) in order to provide callers with free telephone calls. Sometimes several friends would incorporate a black box into each of their telephones to enable them to hold long conversations with each other without having to pay for them. Another use of black boxes was in the incoming modems of computers running bulletin board systems that were popular back in the 1980s and early 90s.