DFRobot GSM / GPS shield with UNO init error
The dfrobot board has a weird boot button system that you need to follow to boot up the board. I spent ages attempting to get it to work before I finally achieved anything.
There is a blue led on the board that will flash continuously at different rates if the chip has been correctly powered and is searching for a mobile network.
I noticed that you commented it the while(!sim808.init()) line. This attempts to power the board on automatically, but I do remember something being funny about the implementation.
From doing a while load of research the dfrobot breakout board is not the best hardware implementation of this chip. Actually from memory there aren't any modules I have been able to find which satisfy all my requirements for a project. I'd recommend looking at all the documentation dfrobot provide on their website. Hopefully that should get you working.
Summary: it's an insanely fiddly process to get it working, but the boards are functional