It's incredible that people actually use these things. Despite everything that we say about the lack of privacy on cell phones people just can't seem to live without them. Imagine, trying to have a private conversation and transmitting it in the radio spectrum. Go figure. Anyhow, since the cellular system is so open, it is the perfect area to be explored by hackers.
People don't seem to realize that cellular phones are incredibly different from regular phones. A cellular phone is actually a radio transmitter and receiver. When talking on a cellular phone, or even when using a normal phone and talking to somone on a cellular phone, the conversation is being broadcast, and can be easily intercepted. Cellular phones do NOT provide privacy.
The average cellular phone is capable of doing a lot more than just being set up for service and used the way the cellular providers intend them to be. Most cellphones have a test mode, where special functions can be accessed. These functions are often as mundane as simply being able to modify the set up for the telephone, modifying the SID, the MIN, and listing the ESN. More interesting functions include being able to use the cellphone as a scanner, scrolling through the cellular frequencies eavesdropping on other people's radio broadcasts.
Things get even more interesting for the hacker when he discovers how easy it can be to clone a cellphone. This allows cellular fraud to happen. (Cellular fraud is a bad thing and causes service providers to lose revenue. It is also illegal! This text is only to show how easy it is to commit fraud on the cellular network, and is not intended to encourage people to use the information for personal gain. Moreso, it is intended to identify to people the risks inherent within the cellular network and encourage the industry to correct these weaknesses.) The key to the cellular system is that each phone is identified by the Electronic Serial Number (ESN) and the Mobile Identification Number (MIN). This information is broadcast from the phone and picked up by the cell tower. The cell tower computer system then cross checks the pair with valid customers and will allow access or refuse access depending upon on the result of the verification. Now since all this information is passing back and forth in the radio spectrum, and no encryption is being used to protect the information, anyone can simply listen in on the proper frequencies and record other people's ESN and MIN codes. Once a valid pair is obtained, it can be reprogrammed into a cellular phone. The newly programmed cellular phone can now be used as if it were the original, and any calls made on it would be billed to the person owning the valid phone with the ESN and MIN pair.
The cellular network can easily be monitored to gather all sorts of information. Once someone has set up a monitoring system, they can watch all the cell calls that are being made. It is trivial to keep track of what cell phones are active, listing both their MIN and ESN. Anyone interested could also keep a log, of what numbers are being called from what cell phones, the time of the call, and the duration of the call (great for both big brother and private investigators). From this information, one could activate automated tape recorders to capture conversations from specific people (Let's say I know Bob's cell phone number. I could set up a system in such a way that each time Bob makes a phonecall, my scanner tunes in to his conversation and records it. Wow, now I'm not just randomly listening to conversations, but I'm picking out whose conversations I want to hear. Won't my friends be surprised.).
Depending on what you want to do, you need different things. If you're interested in doing simple reprogramming of your cell phone, or perhaps turning it into a scanner check out Dr.Who's Radiophone archive. This place is excellent. Dr.Who has put together a huge library covering tons of cellular phones. The information you seek should be here.
If you're looking for software to alter the ESN of a phone you might have to look a little harder, especially depending on the type of phone. Personally, I stick to the Motorola phones myself. For some good Motorola info, and some of the files which might help, check out Mtorola's page.
If you want to set up something of a cell use watchdog site you're gonna have to work at it on your own mostly. You'll need a DDI (Digital Data Interpreter) to understand the signals between cell phones and cell towers. Then you're gonna have to find some software (or build your own) to filter the data. A nice example of this sort of system is made by Electronic Countermeasures Inc. (ECI). Find information on their product by following the link.