Show the actual signal in decibels in place of bars:

• Step 1: Dial *3001#12345#* on your iPhone then press Call.
• Step 2: You'll now enter Field Test mode. You'll notice a signal strength indicator on the upper left hand corner of your screen. You can toggle between signal strength bars and a number by simply tapping on it. At this point, press the home button to exit the app or if you would like the feature to be permanent, continue on.
• Step 3: Hold down on the Power button until you see the "slide to power off" bar. Do not power off the device.
• Step 4: Press down on the Home button until the app closes and you return to your home screen.

That's it. Even when restarting the iPhone, the signal strength can be toggled (by tapping on it) between bars and numbers. The closer the number is to zero, the stronger the signal is.

To return to the default bars permanently, follow these steps:

• Step 1: Dial *3001#12345#* on your iPhone then press Call.
• Step 2: Press on your Home Button once.

• Step 1: Turn on multitasking gestures in the settings menu.
• Step 2: Quit all apps.
• Step 3: Open up Safari.
• Step 4: Use the 5-finger-scroll gesture to scroll to the left.
• Step 5: Hold down on the Power button until you see the "slide to power off" bar. Do not power off the device.
• Step 6: Press down on the Home button until the app closes and you return to your home screen.