Putting iPhone in Field Test Mode allows user to assess signal strength numerically. Field Test Mode also allows user to assess cell tower information, etc through an iPhone interface. Basically, this is the tool that AT&T uses to evaluate area coverage. How do I know? After writing Steve Jobs about my poor coverage, AT&T sent a team to my house and showed me this diagnostic tool -- though they couldn't improve my coverage. When Field Test Mode is activated, the signal bars are replaced by a dBi reading. Around a tower, the reading should be lower (i.e., less negative). It depends on the specs of the tower, but for me, they told me that I should get a -50 reading when near a tower. Gradually the signal will get more negative when you are farther from the tower (unless another tower is picked up). For my situation, AT&T told me that when the reading was around 100, signal strength is poor. (The 100 is not displayed as negative because there is only enough room for three characters.) To access Field Test Mode, go to phone icon, then to keypad and dial *3001#12345#*call. Not sure if this number will appear on the bill, but I doubt it. To Exit, just hit the home button. What are everyone's readings at home? Can anyone else figure out the information give from the other tools that are included?