I don't know about you, but I've definitely had many an occasion where my iPhone has told me that I've had plenty of cell phone signal and yet, empirically, it seemed otherwise. Now I have proof of my suspicions.
[Correction: Thanks to George Ou for pointing out that the signal bars are a pure software construct and under the total control of Apple. My apologies to AT&T for including them in this.]
In order to demonstrate the efficacy of their devices, Wilson had a small Faraday cage at its booth. This cage was a wooden-framed box with copper mesh screening around the insides. Invented by the great Michael Faraday in 1836, a Faraday cage blocks external electric fields from penetrating and this includes RF, or radio frequency emissions such as cell phone signals. Faraday cages are commonly used to create an RF-free environment in which to test RF devices.
While testing my own iPhone 4S in the box we were moderately surprised to see that, some time after the lid had been shut on it, it still had five bars. See the video below, and I apologize for the quality of it, as our real camera operators weren't around and I did it myself.
You can see, if you look carefully, that the cell phone retains five bars even though it has been in the cage for some time. We confirmed that there was no signal in two ways, first by calling my phone while it was in and out of the box. It received calls outside, but not when inside. Also, there is a code you can enter into the iPhone to show the raw signal gain off the antenna and it dropped like a stone when we put it in the box, even as the bars stayed up.
My iPhone gets the job done, but I do find myself frequently irritated by something it does or doesn't do. This, however, is really disturbing, and now you know that you can't trust the bar display on yours.