SIM900 Reply to Incoming Text

Hi,

I wish to have more than one phone Text and reply to each text.
I hav the following code:

// Include Software Serial library to communicate with GSM
#include <SoftwareSerial.h>

// Configure software serial port
SoftwareSerial SIM900(7, 8);

// Variable to store text message
String textMessage;

// Create a variable to store Lamp state
String lampState = "HIGH";

// Relay connected to pin 12
const int relay = 12;

void setup() {
// Automatically turn on the shield
digitalWrite(9, HIGH);
delay(1000);
digitalWrite(9, LOW);
delay(5000);

// Set relay as OUTPUT
pinMode(relay, OUTPUT);

// By default the relay is off
digitalWrite(relay, HIGH);

// Initializing serial commmunication
Serial.begin(19200);
SIM900.begin(19200);

// Give time to your GSM shield log on to network
delay(20000);
Serial.print("SIM900 ready...");

// AT command to set SIM900 to SMS mode
SIM900.print("AT+CMGF=1\r");
delay(100);
// Set module to send SMS data to serial out upon receipt
SIM900.print("AT+CNMI=2,2,0,0,0\r");
delay(100);
}

void loop(){

if(SIM900.available()>0){
textMessage = SIM900.readString();
Serial.print(textMessage);
delay(10);
}
if(textMessage.indexOf("ON")>=0){
// Turn on relay and save current state
digitalWrite(relay, LOW);
lampState = "On";
Serial.println("Relay set to ON");
textMessage = "";
String message = "Alarm is: " + lampState;
sendSMS(message);
}
if(textMessage.indexOf("OFF")>=0){
// Turn off relay and save current state
digitalWrite(relay, HIGH);
lampState = "Off";
Serial.println("Relay set to OFF");
textMessage = "";
String message = "Alarm is: " + lampState;
sendSMS(message);
}
if(textMessage.indexOf("STATE")>=0){
String message = "Alarm is: " + lampState;
sendSMS(message);
Serial.println("Lamp state resquest");
textMessage = "";
}
}

// Function that sends SMS
void sendSMS(String message){
// AT command to set SIM900 to SMS mode
SIM900.print("AT+CMGF=1\r");
delay(100);

//Phone Detailssss
SIM900.println("AT + CMGS = "+447880xxxxxx"");
delay(100);
// Send the SMS
SIM900.println(message);
delay(100);

// End AT command with a ^Z, ASCII code 26
SIM900.println((char)26);
delay(100);
SIM900.println();
// Give module time to send SMS
delay(5000);
}

i am new at this