Now you can put your iPhone into field test mode, which will allow to easily and precisely test your cell signal reception quality. Below are the steps to follow:
• 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.
Info from: 9to5mac.com

