Hi,
you have to use switching Mode DC-DC Converter for at minimum 2A current to make sim800 works. I used LM2576 to powering sim800 in my previous board. because when synchronization with network, sim800 consume lot of power at peak 1-2A burst, yes, even 1A power supply isn't enough.
I have worked with Sim800 device before, and get my board successfully worked. just chat me and let's get started.
regards,
Rijal