http://www.iphonehacks.com/2012/11/how- ... phone.html
How To Check The Actual Signal Strength On Your iPhone
In case you didn’t know, it is possible to see the signal strength indicator in numbers instead of signal bars on any iPhone (except the original iPhone) without jailbreaking it and it’s reversible. It has been one of the old hidden features that works on the iPhone running iOS 4.1 or later.
It’s a lot more useful to see the signal strength indicator as a number [in decibels(dBm)] rather than as a signal bar (after we explain how to read the number a little bit later).
The trick is to enter the Field Test mode on the iPhone and use the trick to force quit an app to exit it.
Here’s how to change the signal strength indicator with a number rather than a signal bar:
Launch the Phone app
Then tap on the Call button, which will launch the Field Test app (or enter the field test mode). You’ll also notice that the signal strength indicator has switched to a number from the traditional signal bar.
Now hold the Power button until the red slider (slide to power off button) appears.
Release the Power button and press the Home button until the Field Test app quits
That’s it. You’ll notice that the signal strength indicator continues to be displayed as a number. You can now tap on the indicator to toggle between the signal strength bar and number. You can revert the changes by entering the field test mode again and exiting the app as you do normally.
Folks at OSXDaily explain how to read the field test signal strength indicator number:
[T]he lower the number (in other words, the more negative) the worse the signal, and the higher the number (less negative) the better.
Anything above -80 is good, and would be considered full bars
Anything below -100 is bad, and would be considered few bars
For example, a signal number of -105 is considerably worse than a signal of -70. You’ll generally find that anything approaching -100 or lower is fairly bad reception, while anything above -80 is usually good, and if you tap the number signal it’s usually shown as full bars. The full range of the signal numbers extends from -40 to -120, with -120 being a nearly impossible number to see because it means no reception, and -40 would be about the strength you’d get being right alongside a cell tower.
You may find this trick useful when you’re in a dead zone and need to find a place with better signal.