Want to be just like that cool guy in the truck who gets to walk around testing cellular reception? Just enter “*3001#12345#*” into your iPhone — you can even do it during a call. This key combination opens the iPhone’s field test mode which is basically a little set or readouts that report on current cellular conditions, firmware information, and all the good stuff that you need to run a wireless company. Can you change anything? Nah, but you can use it to test reception in various areas of your house, like I did, to decide if you need a wireless booster.
Inside the iPhone field test mode [WirelessInfo]