SIM800C: How to make call to other number if first number is busy or no answer

[roypl]

Hi,
Everyone can help me code for sim800c to make call if red led "ON". And if first number didn't give message "yes", SIM800c would call second number

#include <SoftwareSerial.h>

SoftwareSerial mySerial(2, 3);

#define BTN 7
#define RELAY 8

int BTN_state;

void KT_BTN() {
  if (digitalRead(BTN) == LOW) {
    delay(20);
    if (digitalRead(BTN) == LOW) {
      BTN_state++;
      if (BTN_state > 1) {
        BTN_state = 0;
        digitalWrite(RELAY, LOW); // Tắt relay khi nút được nhấn lần thứ hai
        Serial.println("da nhan off");
      }
      if (BTN_state == 1) {
        Serial.println("da nhan on");
        digitalWrite(RELAY, HIGH);
      }
    }
    while (digitalRead(BTN) == LOW);
  }
}

String readSerial() 
{
  delay(100);
  if (mySerial.available()) {
    return mySerial.readString();
  }
}

void CallNumber(String phoneNumber) {
  mySerial.print("ATD+ ");
  mySerial.print(phoneNumber);
  mySerial.println(";");
  Serial.println(readSerial());
  delay(10000);
  mySerial.println("ATH");
  delay(200);
  Serial.println(readSerial());
}

void ReceiveSMS() {
  Serial.println ("SIM800C Read an SMS");
  mySerial.println("AT+CMGF=1");
  Serial.println(readSerial());
  mySerial.println("AT+CNMI=1,2,0,0,0"); 
  Serial.println(readSerial());
}

void setup() {
  Serial.begin(9600);
  mySerial.begin(9600);
  pinMode(BTN, INPUT_PULLUP);
  pinMode(RELAY, OUTPUT);
  digitalWrite(RELAY, LOW);
  BTN_state = 0;
}

void loop() {
  if (digitalRead(RELAY) == HIGH) 
  {
    Serial.println("ON RED LED");
    delay(200);
    CallNumber("+xxxxxxxxxxx");
    delay(2000);
    ReceiveSMS();
    delay(200);
    if (Serial.available() > 1)
    {
      String input = Serial.readStringUntil('\n');
      if(input == "yes")
      {
         CallNumber("+xxxxxxxxxxx");  
      }
    }
  }
  KT_BTN();
}