Now you can put your iPhone into field test mode, which will allow to easily and precisely test your cell signal reception quality. You just need to follow these two easy steps:
• First, upgrade your iPhone 4—or any iPhone—to iOS 4.1.
• Dial *3001#12345#* into your phone.
Done. The signal bars will be replaced with a number (like -85). The more negative the number, the worse the signal is. Example: -110 is worse than -85. Once activated, you can switch between signal bars and number by touching on the number.
Try the test mode now. Then use a finger to connect the two metal bands on the lower left corner, over the black plastic strip. That will short-circuit the magical gap and bring the phone's signal down quite significantly.
To get out of the field test mode, just hit the home button. [Thanks KTK1990!]