How to Avoid a Wiretap

Excellent piece of analysis:

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. Audio examples (in MP3 format) of this countermeasure can be found below.

Loop extender systems are susceptible to other countermeasures as well. In particular, a subject can employ a simple computer-aided dialing procedure (which we call "confusion/evasion dialing") that prevents the dialed outgoing telephone numbers from being recorded accurately by the tap. Wiretap subjects can also falsely indicate the ending times for calls they make and receive and can inject false records of outgoing and incoming calls (appearing to be to or from any numbers they choose) into pen register logs.