Download Blaze’s C tone and broadcast it continuously during phone calls. You can play the tone at low volume so it just seems like ambient room noise...Link
Don’t stop there – befuddle your foes. Play the C for a second in the middle of a call, then without hanging up, dial another number. Analog wiretap systems will interpret this as a new call. You may be chatting with a friend, but now the spooks think you’re talking to Domino’s.
Update: Mark sez, "This would be completely ineffective against the most simple and least effective wiretaps like a wireless transmitter or a voice activated tape recorder attached to your phone line."