According to the Wall Street Journal (pay link), the government (US Marshals Service) is flying “dirtboxes” in small airplanes to capture the signals from your cell phone. Basically, a dirtbox is a self contained cell tower with a strong signal. Since your cell phone will connect to the strongest signal, if the dirtbox winds up being that signal, your phone will connect to that.
Once connected, the feds can grab the ID of that phone (possibly the ESN, IMSI, MIN or similar ID) and the position of that signal and see if it is associated with a bad guy.
In the grand scheme of things, they are not (we don’t think) collecting a lot of data, tracking phone calls or eavesdropping and supposedly once they get this far, they need to get a warrant (according to an article in SC Magazine that references the WSJ article).
Since it appears that the dirtbox is not acting as a real cell phone tower, when you connect to it and try to make a call, that call won’t go through. That would include 911 calls, although the report says that they have taken measures to prevent that – whatever that means.
Since the program is secret (or at least was), we really don’t know the details of the program. Likely, the government would have liked to keep it secret, and they were successful at keeping it secret for 7 years (supposedly it has been running since 2007), but it is hard to keep a secret like that.
One assumes that criminals that watch TV use either burner phones (ones that you can buy at 7-11 or Walmart that are cheap and disposable), but I am sure that some do not.
I am sure more details will emerge.