Except for simple voice inversion, which you can find programs to use a sound card for decrypting, I have never met anyone who has the means to decode anything more complex. There are groups who have contests cracking DES and other types of encryption, but I believe the content is is more data than voice.
I started this thread more than a year ago asking people to decode my encrypted voice. One guy got real close, but I gave him the algo and the DES key. He only got one sentence of it and called it quits.
Good luck, as most agencies will change their key more than once a year.