PROBLEM WITH SIM900A V4

Hello, I need some help guys, I bought a sim900a V4 like this one :

http://www.aliexpress.com/store/product/Sim900a-mini-development-board-gsm-gprs-module-plate-wireless/1021212_1700354759.html

but at the moment I can't have communication with the arduino mega 2560, this is the code that am using:

void setup() { Serial.begin(9600); Serial1.begin(9600); delay(1000); Serial.println("Initializing...");

}

void loop() { if(Serial.available()) { char a=Serial.read(); Serial1.print(a); // Serial.print(a); }

if(Serial1.available()) { char b=Serial1.read(); Serial.print(b); } delay(100); }

have some of you work with this shield?