Caution! : SIM800 module itself uses input voltage of 3.7V – 4.2V. Therefore, if you directly connect the Vcc pin to Arduino 5V pin, it might damage the module as well. I tried 3.3V Arduino pin with no luck. Therefore if you have the SIM800 module itself, you will have to get the input voltage in range, maybe with a LM317 (and few calculations with a LM317 calculator).