Also known as call trace.A CLASS service of the public switched telephone network (PSTN). The feature enables the subscriber to initiate a trace on the last call received. The service is intended as a countermeasure for obscene or harassing calls. The customer initiates the trace by depressing the telephone switchhook and dialing a code.The central office (CO) records the calling number, which can be provided to law enforcement authorities on request, when the called party has filed a proper complaint. See also CLASS and PSTN.