#include <SoftwareSerial.h>

// Makes a SoftwareSerial object in order to communicate with SIM800L module
SoftwareSerial gsmSerial(12, 11); // SIM800L TXD and RXD is connected to the Arduino pins 12 and 11 respectively

void setup()
{
  // Starts the serial communication between the SIM800L module and the Arduino.
  gsmSerial.begin(9600);

  // Starts the serial communication between the Arduino and serial monitor.
  Serial.begin(9600);

  delay(100);
  gsmSerial.println("AT"); // Handshake to check if the module is running. If returns OK, then its fine.

}

void loop()
{
  if (Serial.available() > 0)
  switch(Serial.read())
  {
    case 's':
      SendSMS();
      break;
    case 'r':
      ReadSMS();
      break;

  }

 if (gsmSerial.available() > 0) // if theres any data to collect from the GSM module
   Serial.write(gsmSerial.read()); // then print it to the serial monitor
}