This procedure will change the signal strength indicator on your iPhone from the standard "bars" display to a numeric indicator of the actual signal strength in decibels (dBm). The closer the number is to zero, the stronger your signal is. For example, -75 is better than -110.
Step 1: Dial *3001#12345#* on your iPhone then press Call. This puts your phone in Field Test mode, and you'll notice a signal strength indicator in the upper left corner of your screen.
Step 2: You can toggle between the standard bars indicator and a number by simply tapping on it. Once the indicator changes, press the home button to exit the app, or if you would like the feature to be permanent (until you change it again), continue on.
Step 3: Hold down on the Power button until you see the ‘slide to power off’ bar, but do not power off the device.
Step 4: Press down on the Home button until the app closes and you return to your home screen.
That’s all there is to it. Again, the closer your number is to zero, the stronger your signal is.
To return to the default bars permanently, follow these steps:
Step 1: Dial *3001#12345#* on your iPhone then press Call.
Step 2: Press on your Home Button once.
