Take a few minutes and read the post below this one.
If you absolutely positively must discuss something sensitive over the phone, at least use common sense when and how you do it.
Go to the Radio Shack (preferably in another town) and buy a cheap phone and minutes paying cash. Never ever use any payment method that can be traced back to you.
Do not load any numbers into it.
Do not use it close to your house.
Do not use it unless you're moving - you know, doing 70 mph.
Do not use it more than once.
Use your throwaway to only call other throwaways whose users are following the same precautions. (Thanks to OhioDude for reminding me of that)
When you make your ONE call, remove the SIM card and destroy both the card and phone, then dispose of both of them in separate locations.
Make 'em work for it.