iPhone - see actual signal strength

[Gardener]

So you are trying to make a phone call and it's got 3 bars or 2 bars or whatever. Well, those bars are actually a useless indicator of signal strength. Instead, you want to run your iPhone in field test mode which allows you to see numbers in lieu of the bar system.



Very simple; very neat, as you can see best signal by area, even down to the room in your house!

Enter Field Test Mode on iPhone by dialing these numbers
Entering Field Test Mode on iPhone

This will work on any iPhone model except the original:

   From the iPhone keypad, dial *3001#12345#* and hit "Call"

You will immediately see the signal numbers in the upper left corner, and you can tap around the menus to discover other random features and information that is generally meaningless outside of cell technicians and field operators. If you hit the Home button you'll quit out of Field Test and the signal indicator will return to bars rather than the signal numbers, but it's easy to always see the numbers too as described below.
Enabling Signal Number as Reception Indicator Rather Than Signal Bars

To always see the signal numbers rather than the signal bars, you'll use the Force Quit app function to kill Field Test when it's open:

    Dial *3001#12345#* and hit "Call" if you haven't done so already to launch Field Test
    Now hold down the Power button until the "Slide to Power Off" message appears, then release the Power button and hold the Home button until Field Test quits
    Tap the signal bars or signal numbers to switch between the two

To remove the tap-to-switch signal indicator ability, you can either reboot the iPhone or go back into Field Test and close out of it as usual.
How to Read the Field Test Signal Indicator Numbers

The numbers don't follow a scale that makes much sense to normal people, but the 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 -105 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 -130 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. Technically, the number goes all the way to -140, but you will almost never see that because it basically means there is no signal to speak of, and most users will see -120 or -130 before it switches over to the "No Service" indicator instead.

http://osxdaily.com/2012/08/20/field-test-mode-iphone-signal-strength-numbers/