The accusation has been levied by some that the police (almost all city, county, state, and federal police/enforcement agencies) have a device called "Stingray" that is capable of pretending to be a cellular tower, so that cell phones will connect to it, and thus listening in to phone calls or even recording them. The idea is that they can keep tabs on criminals using this technology, but the technology is not accurate enough to target specific individuals and so all cell phones or random cell phones would be picked up and conversations inspected to determine if the person the law enforcement agency was trying to monitor was the one placing the call. As an example of some of the reporting on this topic, here's a video of an interview with John McAfee about the device, its capabilities, its uses, and software capable of notifying you when your phone has connected to a "Stingray" device rather than a real cellular tower from your cellular service provider:
For those who are skeptical that such a device may exist, then I feel it important to point out that at a popular security conference called DEFCON (specifically DEFCON 18, which took place between July 30th and August 1st, 2010) a live demonstration was made by Chris Paget where he was able to set up a fake cellular tower and force cell phones on the AT&T network to switch over to using it, and even record data going from the phones to the service provider over the Internet connection. Here's the video of that presentation (including technical explanations):
You can also download the full video from the DEFCON website at this link.