This worked perfectly on every version of iOS since version 4.1. But what iOS 7 has brought with its’ many under-the-hood changes has also caused some tweaks to either stop working completely or if we are lucky simply needs to be implemented differently. The -dBm meter happens to be one of them, so if you still want to continue using it follow along as it’s not that much different than before.
Enter Field Test mode by opening the phone app and dialing *3001#12345#* (you can also copy/paste this to make things easier) like any other call. Tap “Call” now your in. You’ll see the signal bars (circles in iOS 7.x) in the upper left corner are now represented by a numerical -dB signal strength meter.
The -dBm meter is a more accurate and meaningful numbered signal strength meter to the circles. To keep it displayed when you leave field test mode do the next two steps to exit Field Test mode.
Hold down the sleep button till the red slider bar appears, the press and hold the home button until the phone restarts. That’s it. You can now display either -dBm or circles by swiping a finger over the signal dislplay to switch between the two.
Disabling the -dBm meter setting from within Field Test mode simply requires you to press the home button to restore default circles.
If you find you really don’t like the -dBm meter after having it enabled. Just dial *3001#12345#* again to renter Field Test mode then press the home button and the default circles will restored.