Even if you can pick up a cell signal from Mars, your building is blocking your phone's signal back to the tower. That's why phones get hot in bad reception areas...constantly trying to transmit at max power. My office has a repeater network. It would have been much cheaper to issue a little antenna to everyone who asked.
If this thing worked, I agree that it would only work in an open air environment. Its allot like the misunderstandings of how WiFi extenders work. While WiFi extenders don't boost your signal, they relay a signal further, this thing claims the opposite. This antenna is attached to the back of your phone, so it is in no way relaying signal to your phone. It is said to be a booster, but if the signal is so poor because you're indoors, how exactly does it get better signal than your phone to begin with?
They sell amplifiers for personal/work use that you can buy, but that involved mounting an antenna in a window or on the roof, running cable to an amplifier, then having an antenna inside. Those stick on or non-powered antenna boosters are all false claims.