String stringXXX;
unsigned long intXXX;
int h;

String messageFromSIMmodule = "Start 1981";

void setup() {
  Serial.begin(115200);
  Serial.println();
}

void loop() {
  
if (messageFromSIMmodule.indexOf("Start") != -1) {
  h = 0;
  while (h < messageFromSIMmodule.length()) {
    if (isDigit(messageFromSIMmodule[h])) {
      stringXXX = stringXXX + messageFromSIMmodule[h]; // add digit to the string
    }
    h++;
  }

  intXXX = stringXXX.toInt(); // convert the extracted string to an integer

  stringXXX = "";
  messageFromSIMmodule = "";

  // Output the result
  Serial.print("Extracted number: ");
  Serial.println(intXXX);
  
  }

  delay(2000);
}