I'm with Hexadecimal on this one. IIRC newer phones are digital and you need something to interpret the signals. I also believe that they transmit on a different frequency now. I know the old cell phones, the bricks, would work in the 800-900Mhz range. I'll have to try screwin around with all my obsolete/broken/collected cell phones and see what they really do.

What I'm more tempted to do, is modify one of my obsolete phones and create one of those cell phone jammers so that no ones phones work within about 10 feet of me.