Some years ago, there was a elderly couple, John and Alice Martin, that used a modified police scanner to listen in on cell phone conversations. They also had a tape recorded hooked up to the scanner so that they could record the conversations they picked up. One day this couple happened to pick up a conversation in which Newt Gingrich (then Speaker of the House) was talking strategy with other Republican leaders. This couple taped this conversation and then turned the tape over to Representative Jim McDermott (D, WA) who then gave it to the New York Times. The Martins were each fined $500 as a result of guilty pleas for their part in this (they could have been fined as much as $5,000 each; http://tinyurl.com/mb8y8). Additionally, one of the other parties involved in the phone call, John Boehner (R, OH), sued McDermott over his role in this and the courts have awarded Boehner $60,000 in damages (http://en.wikipedia.org/wiki/John_Boehner). This particular decision has been appealed. The reason that these three people have gotten into such trouble is because it is illegal to listen in on and record phone calls to which you are not a party. In some state before you can record a phone call one of the parties of the call must consent. In others, all participants must consent. In this case (and in what you are proposing), none of the people involved in the conversation gave permission for any of the activity related to the Martin's eavesdropping. If I remember correctly from when this story first broke, it is also illegal to sell parts with the intent that they be used to intercept other people's conversations. (The Martins had to modify their police scanner themselves.) So, I am not going to tell you how to listen in on other people's cell phone conversations. What I am doing is warning you that it is illegal and that doing so can get you into a whole lot of trouble. Besides, this is a place where the Golden Rule really applies. How would you feel if someone were listening in on your private conversations? If you would not like it, why would you do it to someone else?

Under the Electronic Communications Privacy Act of 1986, it's a violation of federal law to listen in on a cellular phone conversation. It would therefore be a criminal offense to use such a device. There are modifications to some scanners that unblock the cell frequencies (824-894 MHz), but doing the modifications or possessing a modified scanner is also illegal. The best answer to the question is, don't even think about it.

Yes, it's called The PATRIOT Act. Talk to the NSA, they can cue you in on it

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