This gives you a number (sadly, for me, usually ranging between -100 and -80) which is a more precise measure of signal strength. The closer you get to zero, the stronger the signal. Between -50 and -75 is considered excellent. Because of how decibels work, movement from, say, -95 to -90 is a bigger move than -85 to -80 (study up on decibels here or just trust me, closer to zero is better). You can always tap on the decibels and it will revert back to bars.
1. Go into Phone and, as if you were dialing a phone number, hit *3001#12345#*
2. It will go into a field test mode. Tap the decibels/bars in the upper left corner and you can see it switch between them.
3. Hold down the power button (top of iphone) until the “slide to power off” appears.
4. Don’t power off. Now hold down the Home button (circle) until it drops you back to your main home screen.
5. You can now flip between decibels and bars.