Wanna eavesdrop a mobile phone conversation or intercept a text message? That is so "super" illegal. And I am not here to teach you how to do it (believe me, I have never tried it).
It happened recently in Wal Mart where a technician was caught red handed! And based on that incidence
There are a few ways to do this. One method, phone cloning, lets you intercept incoming messages and send outgoing ones as if your phone were the original. If both phones are near the same broadcast tower, you can also listen in on calls. To clone a phone, you have to make a copy of its SIM card, which stores the phone's identifying information. This requires a SIM reader that can read the card's unique cryptographic key and transfer it to another phone. (Warning: This is super illegal, but there are still sites that show you how.) The problem with cloning is that it only lets you intercept messages sent to one phone number. Plus, you need physical access to the target phone to make it work.
It's also possible to intercept unencrypted or poorly encrypted messages directly as they're broadcast over cellular channels. (If the network uses sophisticated encryption, you might be out of luck.) To steal messages with your phone, you would need to upload illegal "firmware" onto your phone. This essentially turns your phone into a radio and allows it to pick up all the texts broadcast on a given channel—instead of limiting you to the ones addressed to you. You'd also need to know the network for the target phone—Verizon, Cingular, T-Mobile, etc.—and you'd have to make sure that both your phone and the target are within range of the same base station. This method isn't too expensive since you don't need much more than a computer, a phone, and some firmware that any serious techie could find online for free.