Editing Blue box (section)

===Countermeasures===
Blue boxing remained rare until the early 1970s when the required systems began to drop in cost and the concept began to be more widely known. At the time, phreakers felt there was nothing [[Bell Telephone Company|Bell Telephone]] could do to stop blue boxing because it would require Bell to upgrade all their hardware.{{sfn|Rosenbaum|1971}}

For the immediate term, Bell responded with a number of blue box detection and law enforcement countermeasures. Armed with records of all long-distance calls made, kept by both mechanical switching systems and newer [[electronic switching system]]s, including calls to [[toll-free telephone number]]s which did not appear on customer bills, telephone security employees began examining those records looking for suspicious patterns of activity. For instance, at the time, calls to long-distance information, while answered, deliberately did not return the electrical "off hook" signal indicating that they had been answered. When an information call was diverted to another number that answered, the billing equipment would log that event. Billing computers processed the logs and generated lists of calls to information that had been answered with an off-hook tone. In the early days, the lists were probably intended to detect equipment malfunctions, but the follow-up investigation did lead to blue box users. After the toll free "800" service was inaugurated, the billing computers were also programmed to generate lists of lengthy calls to toll free numbers. While many of these calls were legitimate, telephone security employees would examine the lists and follow up irregularities.

In this case, filters could be installed on those lines to block the blue box. Bell also would [[wiretap]] the affected lines. In one 1975 case, the [[Pacific Bell|Pacific Telephone Company]] targeted one defendant's line with the following equipment:
* A CMC 2600, a device which registers on a counter the number of times a 2600{{nbs}}Hz tone is detected on the line;
* A tape recorder, activated automatically by the CMC 2600 to record two minutes of telephone audio after each burst of 2600{{nbs}}Hz activity; and
* A Hekemian 51A, which replicates the functions of the CMC 2600 and also produces a paper tape print-out of outgoing calls. Ordinary calls were recorded in black ink and destination numbers called via the blue box were recorded in red ink.<ref>[https://law.resource.org/pub/us/case/reporter/F2/563/563.F2d.967.76-3391.html UNITED STATES of America vs. Bernard CORNFIELD, dba Grayhall Inc], No. 76-3391, United States Court of Appeals, Ninth Circuit. October 27, 1977.</ref>

These actions resulted in several highly publicized trials.