Here’s something I’ve been doing since iOS 4.1 I believe if not earlier. Doing this trick will give you a more accurate signal reading than the meaningless default meter bars. It requires you to put the iPhone into field test mode without any jailbreaking and it’s pretty simple.
To enter field test mode, open the phone app then dial *3001#12345#* as you would any other call (you can also copy/paste this too if necessary). Tap “Call” and your in, that’s it. You’ll notice the signal bars are now represented by a numerical -dB signal strength meter.
From here if you prefer this more accurate and meaningful numbered signal strength meter to the bars just follow these next few steps to keep that displayed when you leave field test mode.
Hold down the sleep button till the red slider bar appears, release the sleep button then press and hold the home button until the home screen appears. That’s it. You can now display both -dBm and bars. Swiping a finger over the signal dislplay will now switch between the two.
To disable the -dBm meter setting if you’re still in field test mode simply press the home button and the default bars will reappear.
If you find you really don’t like the -dBm meter at all and already have it enabled. Just dial *3001#12345#* again to renter field test mode then just press the home button and the default bars will reappear again.