sahniana:
using LM7805 connected to diode 1N4007, i managed to power up this module. but as you can say, i can't access anything related to SIM, such as AT+CNUM command, it will restart the module, i think it related to power shortage.
7805 is grossly unsuitable for SIM800. SIM800 requires peak current of upto 2A while most 7805 in market these days provide a measly 300-400mA (as compared to theoretical 1A)..
you should try supplying power through a switching regulator like 2576 or LM2596.
Else, buy a SIM800 with onboard voltage regulator. SIM800 is a great GSM Modem and has been used to replace SIM900A by SIMCOM. if you are in India, I can suggest couple of places to buy GSM Online.