There are several commercially available "RF service monitors" with option modules specifically designed for AMPS (the North American cellular standard). Manufacturers include IFR, HP and Marconi. Among many other things, these monitors can be programmed to monitor cellular access channels. Whenever someone nearby hits the SEND key on their phone, the monitor instantly displays the called number, the user's MIN (phone number) and ESN (electronic serial number). Furthermore, it can be told to automatically follow the conversation channel assignment message and any subsequent handoff messages. Or the unit can be programmed to monitor the forward paging link for pages (land-to-mobile) calls directed to any particular mobile. When a page is found, the unit can again switch to the appropriate conversation channel and follow the conversation through any subsequent handoffs. You do, of course, have to remain physically close enough to the mobile in question to be able to hear the same cell sites it is using. As a manufacturer of cellular phones, we have legitimate need for such units in testing our phones. We had one of these units in house a while back and I had a chance to play with it. I can attest to its effectiveness. It's not cheap, of course, but if we can afford one, than so can any motivated government agency. Phil