I read about this awhile back.
http://freerepublic.com/focus/f-news/3290276/posts
Stingray masks itself as the strongest tower in the area, forcing your phone to make it the default choice, then once it has control of your signal, forces it into unencrypted mode, where anybody can listen in.
Kinda creepy. Yet these units are for sale to the public?!
I don’t think you can buy one as a citizen. To me this is purely illegal government sh!t.