So apparently some security researchers have figured out how to disable CALEA wiretaps using a very simple technique that has been around since the very early days of phone system hacking (phreaking to us the hacker jargon). By emitting a C-Tone on the line during the call you can confuse the wiretapping software into thinking that the phone is on the hook and thus it will not record the audio. The paper below also details how you can confuse the system as to what number you dialed and how long the call lasted.

http://www.crypto.com/papers/wiretapping/