Sometimes, we maybe be curious to know, how the signal strength is provided by your iPhone provider, at a particular location. Currently in Netherlands where I live, initially iPhones were provided exclusively by T-Mobile NL and only recently providers like KPN and Vodafone are providing it.
So, there is an undocumented way to know how much the signal strength is for a given provider.
The utility is called Field Test, and it is accessed by dialing *3001#12345#* (followed by the "Call" button).
When you do that, another page launches with a title bar that reads "Field Test," along with a Refresh button, as you can see in the image below. The Field Test part is that your signal bars will be replaced with a negative number that measures signal strength as expressed in decibels of noise in the signal.
As you see from the screenshot above, what this means, is that on my iPhone 4 (iOS 7) my signal strength is at –80db at my home. The lesser this value, the better is your signal strength.
I still have not figured out what the other options like UMTS cell environment does, but when I discover then, I'd explain this in my next posting.
Pressing the Home button on your iPhone will end the Field Test and return your display to normal.