PROBLEM WITH GSM

I am using a SIM900A GSM Module for my project on arduino based vehicle tracking system using gps and gsm. Just to check the working of the GSM, I connected the Tx and Rx pin of the GSM to arduino Rx and Tx pins respectively, and connected the GND pin of the GSM to that of Arduino. I am using an Arduino UNO board. For powering up the GSM I am using a 12v 2A adapter. When I am powering on the GSM module, the DC-PWR led is turning on. But the Status LED is not turning on at all and the NWK LED is blinking at every 1 sec interval approx. Its not registering on network when a SIM card is inserted, nor it is responding to any of the inbuilt examples that I upload. I have tried all the SIM Cards that I have. They are all unlocked and not protected by PINCODES. Whats the problem? Is it faulty? A help will be appreciated. The code that I used is given below.

/*
 SMS sender

 This sketch, for the Arduino GSM shield,sends an SMS message
 you enter in the serial monitor. Connect your Arduino with the
 GSM shield and SIM card, open the serial monitor, and wait for
 the "READY" message to appear in the monitor. Next, type a
 message to send and press "return". Make sure the serial
 monitor is set to send a newline when you press return.

 Circuit:
 * GSM shield
 * SIM card that can send SMS

 created 25 Feb 2012
 by Tom Igoe

 This example is in the public domain.

 http://www.arduino.cc/en/Tutorial/GSMExamplesSendSMS

 */

// Include the GSM library
#include <GSM.h>

#define PINNUMBER ""

// initialize the library instance
GSM gsmAccess;
GSM_SMS sms;

void setup() {
  // initialize serial communications and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }

  Serial.println("SMS Messages Sender");

  // connection state
  boolean notConnected = true;

  // Start GSM shield
  // If your SIM has PIN, pass it as a parameter of begin() in quotes
  while (notConnected) {
    if (gsmAccess.begin(PINNUMBER) == GSM_READY) {
      notConnected = false;
    } else {
      Serial.println("Not connected");
      delay(1000);
    }
  }

  Serial.println("GSM initialized");
}

void loop() {

  Serial.print("Enter a mobile number: ");
  char remoteNum[20];  // telephone number to send sms
  readSerial(remoteNum);
  Serial.println(remoteNum);

  // sms text
  Serial.print("Now, enter SMS content: ");
  char txtMsg[200];
  readSerial(txtMsg);
  Serial.println("SENDING");
  Serial.println();
  Serial.println("Message:");
  Serial.println(txtMsg);

  // send the message
  sms.beginSMS(remoteNum);
  sms.print(txtMsg);
  sms.endSMS();
  Serial.println("\nCOMPLETE!\n");
}

/*
  Read input serial
 */
int readSerial(char result[]) {
  int i = 0;
  while (1) {
    while (Serial.available() > 0) {
      char inChar = Serial.read();
      if (inChar == '\n') {
        result[i] = '\0';
        Serial.flush();
        return 0;
      }
      if (inChar != '\r') {
        result[i] = inChar;
        i++;
      }
    }
  }
}