iPhone Internal Diagnostic utility

Tested on iOS 4.2 @ 4.3.5

Launch the dial pad and enter:
*3001#12345#*

.. then press Call.

Once in the field test screen you can view a bunch of different internal information and if you click on the Signal bars you will actually see the signal in numerical value. The closest to zero (0) you are the better signal.

To get out simply press the Home button.

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