In a research paper appearing in the November/December 2005 issue of IEEE Security and Privacy, we analyzed publicly available information and materials to evaluate the reliability of the telephone wiretapping technologies used by US law enforcement agencies. The analysis found vulnerabilities in widely fielded interception technologies that are used for both "pen register" and "full audio" (Title III / FISA) taps. The vulnerabilities allow a party to a wiretapped call to disable content recording and call monitoring and to manipulate the logs of dialed digits and call activity.
In the most serious countermeasures we discovered, a wiretap subject superimposes a continuous low-amplitude "C-tone" audio signal over normal call audio on the monitored line. The tone is misinterpreted by the wiretap system as an "on-hook" signal, which mutes monitored call audio and suspends audio recording. Most loop extender systems, as well as at least some CALEA systems, appear to be vulnerable to this countermeasure.
