So you're saying that you CAN increase signal strength passively, just not in this way, right? If you've got the right shape and size of antenna, you should be able to. At least I've always seen it work with my TV, radio, and wifi station.
No, I just didn't want to write a dissertation on radiators, signal to noise, and other radio topics so I was too brief.
You can focus a signal of given strength so that more signal is directed at the antenna of the main system. A large enough reflector will (in a mobile device such as a phone) block the antenna just as often as it focuses more signal at it. A flat reflector is minimally effective, you want a properly curved reflector that focuses reflected signal at the main antenna. And tracks the cell tower and cellphone, constantly adjusting angle to keep them focused on each other.
When you start reflecting signals you start running in to distortion, multi-path and other signaling issues that may or will counteract the extra radiation directed at the antenna and will probably degrade overall call quality.
Angle of incidence = angle of reflection. So this thing should only, theoretically, reflect an extra "signal" of the size of the built-in antenna. That reflected signal will arrive at the antenna slightly after the antenna received the directly transmitted signal. That does not mean you doubled signal reception strength.
All that aside, there's this:
A cell phone to cell tower conversation is two ways. If this thing is absorbing signal that would otherwise have missed the phone's antenna and focusing it at the phone's antenna then it must, in reverse, also be absorbing the cell phone antenna's output signal and diffusing it over the larger area of the "booster" and spreading it out more so the cell tower can't hear the phone as well.
I mean... how is this thing focusing/amplifying output from the phone?
I know this is America and in this country we always think "bigger is better", "more is better", but when dealing with antennas there are physics involved that dictate antenna size. Simply adding more antenna will not necessary make things better.