Folks at WirelessInfo.com have published a number which if dialed brings up the iPhone Field Test Mode, which will give you a peek in to AT&T's network by providing information about the cell tower in your area, signal strength, EDGE network, and lots more.
So how do you get into the Field Test mode on your iPhone??
iPhone Field Test mode - Main Screen
Before I give you the number, let me first provide you the warning published by Richard Baguley of WirelessInfo.com.
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.
As can be seen in the images below; the iPhone Field Test mode shows a lot of information, supposedly more than many other phone field mode. One cool thing about this number is it can be used even when you are in a call.
Here is the number, to access the iPhone Field Test modes, dial *3001#12345#*. (Remember the warning and that you are doing it at your own risk)
An observation by one of those who tried so that you don't end up hitting the same issue; Classicuser at MAC OS X hints mentions:
Oddly, if you dial this directly on the keypad, it seems to work and provide the data indicated. But, if you add this number to your contacts list and select it from there, it results in the "Your call cannot be completed as dialed" recording.
Please find the images below which gives you a visual walk through of the options available on the iPhone when you switch to Field Test mode as one navigates through the menu options.
iPhone Field Test mode - Main Screen
Network Information
Cell Information 1
Cell Information 2
GPRS Information
Call Information
Versions
[Source and Images courtesy WirelessInfo.com]
Some of the readings in above snapshots are away from being real, are those snapshots taken from real network if so i do beleive that the field test is un true, for example there is no such as AFRCN 138 in GSM system, C1,& C2 cannot have the values of -32768, incase of DTx is off the rxqual sub and full should have the same values where its not!
Not only that, but the graphics are all now wrong, since the field test screens changed with the 2.0 firmware update.
Nut case–this hasn’t worked since the old 3.12 firmware. You must be stuck in some bad trip 4.1 you smoking some bad stuff!