sim800A not registring on network

I am using sim800A interfacing with Arduino UNO

Connections:
GSM == Arduino UNO
RX – TX
TX – RX
GND – GND

power supply: 12v 2A adapter

sim card: BSNL card

Where is the problem…?

I am getting led status blinking every second…!!

Antenna and power connection topology, regulator used to convert 12V to 4V ?

No, i am using 12v 2A ac-dc adapter

Modules operate on 4V input. How are you reducing the adapter voltage to module voltage?

I was directly using the adapter…
Can you suggest how to go…?