GSM_Number_Auth.ino

#include <SoftwareSerial.h>

//Create software serial object to communicate with SIM800L
SoftwareSerial mySerial(3, 2); //SIM800L Tx & Rx is connected to Arduino #3 & #2
 String callerID;
 int flag = 0;
void setup()
{
  //Begin serial communication with Arduino and Arduino IDE (Serial Monitor)
  Serial.begin(9600);
  
  //Begin serial communication with Arduino and SIM800L
  mySerial.begin(9600);

  Serial.println("Initializing..."); 
  mySerial.println("AT"); //Once the handshake test is successful, it will back to OK
  updateSerial();
  mySerial.println("AT+CLIP=1"); //To get caller ID
  updateSerial();
  
}

void loop()
{
  updateSerial();
}

void updateSerial()
{
  delay(500);
  while (Serial.available()) 
  {
    mySerial.write(Serial.read());//Forward what Serial received to Software Serial Port
    
  }
  while(mySerial.available()) 
  {
    callerID = mySerial.readString();  //Read serial and make a string 
    if(callerID.indexOf("9747165032")>0){ //compare with a defined number
      flag = 1; 
      Serial.println("Registered"); 
      mySerial.println("ATH"); //to Refuse call 
      //Execute your loop here.
      
    }
    else if(flag){ //To avoid multiple time of execution 
      Serial.println("Not Registered");
      flag = 0;
      
    }
  }
  
}