sim900A A6 GPRS/GSM
The SIM900A and the A6 are made by different companies. You need to be clear on which device you have. Link?
We also need a drawing of how you have it connected up. Do you have the device attached do D0 & D1, AND you are using serial monitor?
What I always suggest in cases like this is upload a simple serial relay sketch to the Arduino, then send the AT commands manually and see what response you are getting.