Check the power supply. The SIM800 sometimes draw more than 1 amp, like when you start a call.
I test 2 diferent modules, put a common use diode, it draws about 0.7 volts, and atach to a 5 volts with 2 amps min. I test it with a computer power supply. Dont conect to 5v directly unless the board have voltage regulator!! it fries the module.
I have a lot of problems when i start to test it, but with that conections and the diode i can make calls and send sms.
With the module conected to a 5v like the Arduino power supply, it power up, but NEVER connect to a network, maybe you can comunicate with it with AT commands, but it will reply only symbols or ERROR messages.