Here are instructions on how to replace the signal strength bars on iPhone with a numerical representation of reception strength. (ie. numbers instead of bars).
First select to launch MobilePhone from your Springboard.
Tap the keyboard tab then dial *3001#12345#*.
Now your iPhone will 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.
You are required to 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.
You must 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.