GSM GPRS sim900 shield doesn't work properly afte turning it on

Other Hardware General Electronics
12

This loops over the ultrasonic sensor 20 times. Try adding something like this:

#include <SoftwareSerial.h>
#define trigPin 2
#define echoPin 3
// Configure software serial port
SoftwareSerial SIM900(7, 8);
//Variable to save incoming SMS characters
char incoming_char=0;
String message;
int led = 9;
int sirena = 5;

int timer = 0;
int timer2 = 0;

long duration;
int distance;


void setup() {
 SIM900.begin(19200);
Serial.begin(19200); 
// Give time to your GSM shield log on to network
  //delay(20000);
 // AT command to set SIM900 to SMS mode
  SIM900.print("AT+CMGF=1\r"); 
  
  // Set module to send SMS data to serial out upon receipt 
  SIM900.print("AT+CNMI=2,2,0,0,0\r");
  
  pinMode(led, OUTPUT);

  pinMode(sirena, OUTPUT);
  //digitalWrite(led,LOW);
  pinMode(led2, OUTPUT);
   pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}
void loop() {
  if (timer == 1){
      timer2++;
      if (timer2 == 20){
          digitalWrite(trigPin, LOW);
          delayMicroseconds(5);
          digitalWrite(trigPin, HIGH);
          delayMicroseconds(10);
          digitalWrite(trigPin, LOW);

          duration = pulseIn(echoPin, HIGH);
          // Calculate the distance:
          distance = duration * 0.034 / 2;
          Serial.print("Distance = ");  
          Serial.print(distance);
          Serial.println(" cm");

          timer2 = 0;
          timer = 0;
      }
  }
  else {
      digitalWrite(trigPin, LOW);
      delayMicroseconds(5);
      digitalWrite(trigPin, HIGH);
      delayMicroseconds(10);
      digitalWrite(trigPin, LOW);

      duration = pulseIn(echoPin, HIGH);
      // Calculate the distance:
      distance = duration * 0.034 / 2;
      Serial.print("Distance = ");  
      Serial.print(distance);
      Serial.println(" cm");
  }
  if(SIM900.available() >0) {
    message = SIM900.readString();
    timer=1;
    //Get the character from the cellular serial port
  
  }else
{
  message = "";
}
   if(message.indexOf("+381601416081")> -1){
  if(message.indexOf("ON") > -1){
      Serial.println("LED ON");
     digitalWrite(led, HIGH);
}
    else if(message.indexOf("OFF") > -1){

      Serial.println("LED OFF");
      digitalWrite(led,LOW);
}

  
}
  
 if( distance < 20 && digitalRead(led) == HIGH){
  SIM900.println("ATD + +381601416081;");
  
  SIM900.println();
  digitalWrite(sirena, HIGH);
  delay(300);
  digitalWrite(sirena, LOW);
  
  
}
}