SMS Receiving problem - Arduino Uno and SIM900A GSM Module
Instead of using At commands use the GSM library of Arduino it easier to understand and implement.
the code to recieve SMS

    #include &lt;GSM.h&gt;

    // PIN Number for the SIM
    #define PINNUMBER "" //sim pin incase the sim is kept locked 

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

    // Array to hold the number a SMS is retreived from
    char senderNumber[20];

    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 Receiver");

      // connection state
      boolean notConnected = true;

      // Start GSM connection
      while (notConnected) {
        if (gsmAccess.begin(PINNUMBER) == GSM_READY) {
          notConnected = false;
        } else {
          Serial.println("Not connected");
          delay(1000);
        }
      }

      Serial.println("GSM initialized");
      Serial.println("Waiting for messages");
    }

    void loop() {
      char c;

      // If there are any SMSs available()
      if (sms.available()) {
        Serial.println("Message received from:");

        // Get remote number
        sms.remoteNumber(senderNumber, 20);
        Serial.println(senderNumber);

        // An example of message disposal
        // Any messages starting with # should be discarded
        if (sms.peek() == '#') {
          Serial.println("Discarded SMS");
          sms.flush();
        }

        // Read message bytes and print them
        while (c = sms.read()) {
          Serial.print(c);
        }

        Serial.println("\nEND OF MESSAGE");

        // Delete message from modem memory
        sms.flush();
        Serial.println("MESSAGE DELETED");
      }

      delay(1000);

    }