Press to launch MobilePhone from your Springboard.
Tap the keyboard tab then dial *3001#12345#*.
Your iPhone will now launch Field Test. Notice your strength bars have been replaced with a number. This number is your reception strength measured in decibels. The closer to 0dB the stronger your signal.
Press and hold the Power button until the power slider appears.
Press and hold the Home button until the Field Test program is terminated. Notice the signal strength numbers still remain.
To Switch Back to Bars
Dial *3001#12345#* again to enter Field Test mode.
Press the signal strength at the top left of screen to toggle between numbers and bars. Then press the Home button to exit Field Test mode.