Problem connecting Arduino Uno R3 to SIM900 GPRS/GSM shield and sending an SMS

Hardware Arduino GSM Shield
9

Serial Relay sketch

// Serial Relay - Arduino will patch a 
// serial link between the computer and the GPRS Shield
// at 19200 bps 8-N-1

#define RxPin 7
#define TxPin 8

#include <SoftwareSerial.h>

SoftwareSerial GPRS(RxPin, TxPin); // RX, TX

void setup()
{
  pinMode(13, OUTPUT); // Turn off pin 13
  digitalWrite(13,LOW);  
  GPRS.begin(19200);
  Serial.begin(19200);
}

void loop()
{
  if (GPRS.available())
  {
    while(GPRS.available())
    {
      Serial.write(GPRS.read());
    }
  }
  if (Serial.available())
  {
    while(Serial.available())
    {
      GPRS.write(Serial.read());
    }
  }
}

Open serial monitor and make sure the baud rate is set to 19200. Type AT (return) and it sound echo in the main screen AT OK.