GSM MODULE-How to determine Valid center!

i have made one project that controlls the relay through GSM-SIM900A and Arduino UNO.

I made one program and its work satisfactory.

#include <SoftwareSerial.h>

SoftwareSerial GPRS(10, 11);
String textMessage;
String relayState = "LOW";

const int relay = 4;
const uint8_t switchPin = 2;
const bool switchOn  = false;     // using INPUT_PULLUP
const bool switchOff = true;
bool lastState = switchOff;
bool newState;
int joystick[1];
void setup()
{
  pinMode(relay, OUTPUT);
  /*digitalWrite(relay, HIGH);*/
  pinMode ( switchPin, INPUT_PULLUP );
  GPRS.begin(9600);
  Serial.begin(9600);
  GPRS.println("AT+CMGF=1");
  delay(500);
  GPRS.print("AT+CNMI=2,2,0,0,0\r");
  delay(100);
  
}

void loop()
{
  
  joystick[0] = digitalRead(switchPin);
  newState = joystick[0];
  
  if (newState == switchOn and lastState == switchOff)
  {
   
    sendSMS(); 
  }

  lastState = newState;
    delay(500);
  if (GPRS.available() > 0) {
    textMessage = GPRS.readString();
    Serial.print(textMessage);
    recvSMS();
    delay(10);
  }
}
void recvSMS() {
  if (textMessage.indexOf("on") >= 0) {
    // Turn on relay and save current state
    digitalWrite(relay, HIGH);
    relayState = "on";
    Serial.println("Relay set to ON");
    textMessage = "";
  }
  if (textMessage.indexOf("off") >= 0) {
    // Turn off relay and save current state
    digitalWrite(relay, LOW);
    relayState = "off"; 
    Serial.println("Relay set to OFF");
    textMessage = "";
  }
if(textMessage.indexOf("state")>=0){
    String message = "relay is " + relayState;
    sendSMS(message);
    Serial.println("relay state resquest");
    textMessage = "";
}
}
void sendSMS() {
  Serial.println("Switch was turned ON ");
  GPRS.println("AT+CMGS=\"+917987553768\"");
  delay(500);
  GPRS.println ("Switch was turned ON");
  GPRS.write( 0x1a ); // ctrl+Z character
  delay(500);
}
void sendSMS(String  message){

  Serial.println(message);
  GPRS.println("AT+CMGS=\"+917987553768\""); 
  delay(500);
  GPRS.println(message);
  GPRS.write( 0x1a );
  delay(500);
}

how can i insert valid sender in this program for determine the phone number which sent the incoming message for actuating relays.

could you please help.