Stingrays have been reported on widely within the last few years, I strongly encourage you to read everything you can about them; They are perhaps the most heinous threat to civilian privacy in america today.
If you are not familiar, a stingray is quite simple in principle. It is a device that imitates the activity of a cell phone tower, tricking phones in the vicinity into connecting to it. Calls and texts that pass through are saved, along with metadata relating to the device of origin, for the owner of the stingray to peruse at their leisure.
Sophisticated stingrays can even flash a phones firmware, making it possible to intercept conversations as if the infected phone was a bug. These devices are quite simple to build, and could be assembled by a hobbyist for a few hundred dollars.
The local police forces of the United States readily implemented this technology as soon as it was available. Minnesota was one of the first states to implement stingray devices.
Initially, stingrays could only be used as a means of gathering evidence against serious criminals; court orders and warrants were to be collected beforehand. Unfortunately, stingray evidence has cropped up in cases as petty as possession of a small amount of marijuana.
Currently, feds have to get a warrant to use a stingray in any case, but here's the kicker: that doesn't apply to local police forces, the most prolific users of the technology since its inception.
Are you comfortable with the contents of your phone being open to any faceless mook with a badge? I know i'm not.