Serial.available is giving data even input data is not coming

Question

Dear friends I have written a code to identify the caller and send a reply SMS to them using sim 800L and pro mini. My program is to identifying the caller number but continuously sending the SMS in an infinite loop. I am not getting what is the reason, kindly help me, thanks in advance. Code is attached

#include <SoftwareSerial.h>
SoftwareSerial Serial1(4,3); 
String a;
String CellNumtemp;
String CellNum;
void setup()
{
  Serial.begin(9600); 
  Serial1.begin(9600);
  Serial1.println("AT");
  delay(1000);
  Serial1.println("AT+CSMP=17,167,0,0");
  delay(1000);
  Serial1.println("AT+CLIP=1");
  delay(1000);
   }
void loop(){
   if (Serial1.available() > 0){
   String a = Serial1.readString();
   String  CellNumtemp = a.substring(a.indexOf("+91"));
    CellNum = CellNumtemp.substring(0,13);
     Serial1.println("ATH");
     delay(500);
     Serial.println(CellNum);
     delay(500);
    if (a.indexOf("+91") >= 0){
        sendSMS();
       }
     }
  CellNum = "\0";
}
void sendSMS(){  // Function that sends SMS
     Serial1.println("AT+CMGF=1\r"); 
     delay(1000);
     Serial1.print("AT+CMGS=");
     Serial1.print("&quot;");
     Serial1.print(CellNum);
     Serial1.print("&quot;");
     Serial1.println("\r"); // to send SMS
     delay(1000);
     Serial1.println("Happy to see your call. I am good"); 
      delay(1000);
     Serial1.println((char)26);
     delay(1000);
     Serial1.println();
     delay(5000); 
     Serial1.flush();
     delay(2000);
    }