activate iPhone field test mode
Got this from http://www.wirelessinfo.com/content/...-test-mode.htm
Want to take a look at what's going on inside your iPhone? Dial *3001#12345#* and hit Call and you'll get access to the special field test mode of the iPhone. This mode is designed to allow technicians and Apple reps to get information on the inner workings of the iPhone and the cell phone network it talks to.
[Field test code via MacOSXHints]
NOTE: Although it seems that most of the information is read-only (so you can't change anything), field modes like this have the potential to damage your phone and possibly interfere with the phone network. We are providing this information as-is; we cannot be held responsible if anything you do in this field mode damages your phone or the phone network.
The iPhone field mode shows a lot of information. In fact, it is more comprehensive than many other phone field modes, allowing you to see the details of the individual cell towers and a lot of detail about the cell phone network. To access it, dial *3001#12345#*. If you are already in a call, just hit "add call", enter the number above and hit call; the phone will go into test mode, but keep your call connected.
For a phone that tries to hide much of the technical details of how it works, that's a litte surprising. [main screen] The first screen that you see is the main menu for the field mode. This presents you with the following choices: Network Information, Cell Information, GPRS Information, PDP information, Call Information and Versions. Let's look at each of these in turn.
The network information screen shows a large number of technical details on the cell phone network you are connected to. This is, of course AT&T (unless you've managed to hack the phone, in which case we'd love to hear how). Most of these things are only of interest to cell phone engineers and AT&T technicians, but the particular things of interest are RX LevFull, which shows the received signal strength if you are in a call. TX Powr would seem to be the transmitting power, but it isn't clear what that number means.
This is an interesting one; cell information shows the details of the cell phone towers that the iPhone can currently see. The items listed by each cell site are RX (the received signal stength), the FQ (frequency band), ST (station id), CI (the cell tower ID), N (the network ID) and # (a status code).
You can click on any of the listed cell towers to get more details (as shown above), such as the network ID (which will be mostly 410; that's the code for AT&T), the cell ID (each tower has a unique ID) and the strength of the signal it is receiving from that tower. AT&T technicians can use this data to analyze the network and determine if there are any weak spots; you can use this data to look at what cell towers are nearby and how strong the signal is. Remember that the iPhone is locked to AT&T, though, and that this won't show the cell towers for non-GSM networks like Verizon and Sprint.