How to Replace your iPhone Signal Strength Bars with Numbers
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).
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.