There seems to be a lot of contradictory information going around about how to fiddle with one of these things. Instead of taking sides (and possible being WRONG), I will just present here some of the information I found. If you find a procedure that works for a particular computer / phone / software version, mail me and tell me and I will put it up here.
All of the ESN changing schemes require a reprogramming cable to be built. The most difficult part to obtain for this cable is the plug that goes into the bottom of the phone. Everyone I know of has cannibalized one of the cheaper cigarette-lighter adapters for its cord. The problem here is that not all adapters are alike. Some manufacturers sensibly left out the pins of the connector that they would not use. Thus, you must either be able to inspect the cord/plug and see that it has all the proper pins, or be prepared to hack the proper modifications into one that doesn't. If your cable needs pins, use "header" pins - available from Radio Shack as "wire wrapping posts" (part # 276-1987, $1.19/20 ). When I find out which models of lighter adapters are good for this project, I will put a list on this web page.
The rest of the electronics is not too difficult for anyone with a little experience with a soldering iron to put together.
The next issue is the computer. Evidently the software was written a few years back when most computers ran at clock speed of less than 20 MHz. Due to poor program design, the timing loops are hard-coded. This means that it is quite easy to have a computer that is TOO FAST for this project. Sources I trust tell me that systems with clock speeds of between 10 and 20 MHz work, with a 16MHz 386 being optimal. Other sources have reported success with 25 MHz and higher speeds. The next issue is the system's parallel port. It must be bi- directional. Most systems of modern vintage do have bi-directional parallel ports, though some must have the settings in the CMOS setup changed. Of course, the system must also run MS-DOS and be at least somewhat IBM- compatible (whatever that means). Some of the software requires that the config.sys has FILES=8 or higher.
There has been major advances in reprogramming software in the past few years. For a long time the only software was the 'old reliable' mot6.6 program. This is still a very useful program. However, it will only program phones with firmware versions of 9121 or earlier. I have seen two different packages of this software floating around the net and BBSes, and have noticed no appreciable difference. Other knock-offs of this 6.6 software seem to be the 9.9 and 9.11 versions. They have been said to be able to reprogram phones of version higher than 9121, but the author has never seen this succeed. Other respectable sources are of the opinion that these versions are nothing more than utter frauds, hacked up versions of mot6.6 with the version numbers changed. So use mot6.6 and standard flip programming cables for phones with firmware older than 9122. For newer phones, you MAY be able to simply acquire older firmware and install it in the phone. I doubt this works on all but the phones with firmware just after 9122, say up to 9130 or so. For phones newer than that, you should just hope that the phone is not 95XX firmware version or newer. I know of no way to reprogram the ESN on these with a computer and cables, though some rumors of COPY-CAT boxes being able to reprogram these phones have been heard but not verified. For phones between 9122 and 95XX, there are a few methods. First, try the CF-10 EPROM, then the New Loader EPROM, and then the G2 program. The G2 software and special ID transfer cables are required, as is an older (pre 9122) Motorola "bag" phone which has the special LOADER firmware installed. This scheme works by first reading the ESN out of the flip-to-be-programmed and puts it in the "loader" phone, where G2 changes it, then the ESNs are switched back again. Voila! the phone is now cloned. This is a very tricky process. There may be many complications. This process is not for the technologically challenged or mentally impaired. Do not try this with a phone that you can't bear to lose until you have practiced enough and are sure you have the process correct. I will be updating this page with hints and corrections as I come across them. Do not mail me with questions, because if it's not here I don't know it. If you have corrections, tell me. If you fuck up your phone, don't say that I didn't warn you! There are no guarantees of the correctness of the data presented here, or anywhere else in the radiophone pages.