When I used my iPhone 6 or iPhone 5S with iOS 8 or older, I could easily see the signal strength as a number in decibels (dBm) rather than as a signal bar. It is shown in the upper-left corner after entering Field Test mode.
However, when I moved to an iPhone 6S Plus running iOS 9, the number seems to disappear from the screen. So, how to see the numerical signal strength again?
First, you still need to open the Field Test mode by dialing *3001#12345#* in the Phone app. The signal number is actually still there but it is hidden. You will have to scroll the notification bar down to see it.
In case you want your iPhone to display the signal number instead of the signal bar, you’ll need to press and hold the Power button until the “Slide to power off” screen shows up. Then, press and hold the Home button for a few seconds to come back to your iPhone homescreen.