Keep in mind this is about tracking based on Numerical probability just like a scenario from the TV Show Numb3rs.
They are not looking at listening to your conversations, but that might come in the form of a warrant after the probability suggests you are now a suspect.
Everyone should be aware that there are devises available for individuals to scan the frequencies and lock in on your call any time they want.
I used to have a scanner from Radio Shack that would allow me to listen to all of the cordless phones in every house for almost a mile. It was illegal to do so, and so I never did as far as you know.
I'm surprised I didn't see the argument that the air waves are controlled and regulated by the Government and therefore thare should be no expectation of privacy.
I also want to restate for those who might have missed it before. If you text message you can delete your ability to even see a message but it stays in the Phone Company server essentially for ever, and the same is true of all E-mail. So be careful of what you say and to whom you say it, especially if you cheat on you spouse.