Interfacing Mega 2560 With M590 GSM Module

Hello Kindly Assist been having major challenge on communication between Mega 2560 and M590E GSM Shield. I have tried various codes but the gsm won't send an sms neither receive. could my wiring be wrong.

#include<SoftwareSerial.h>      
SoftwareSerial firstSerial(0,1);  //RX/TX    
int led=13;    
void setup()      
{      
  pinMode(led,OUTPUT);    
  firstSerial.begin(9600);      
  Serial.begin(9600);      
  delay(100);      
}      
void loop()      
{      
  if(Serial.available()>0)    
  digitalWrite(led,HIGH);      
  switch(Serial.read());      
}      
void sndmsg()      
{    
  firstSerial.println("AT+CMGF=1");      
  delay(1000);      
  firstSerial.println("AT+CMGS=\"0963645551\"\r");  //Enter Your Mobile Number instead XXXX while Testing    
  delay(1000);      
  firstSerial.println("Hello C# Corner Iam Sending SMS Through GSM");      
  delay(100);      
  firstSerial.println((char)23);      
  delay(1000);      
}      

what’s char 23 ?

Double check before you answer.

Hi Sorry made a mistake when copying the exact code


// Include DHT library and Adafruit Sensor Library
#include "DHT.h"
#include <Adafruit_Sensor.h>
//Include Software Serial library to communicate with GSM
#include <SoftwareSerial.h>

// Pin DHT is connected to
#define DHTPIN 12
  
// Uncomment whatever type of sensor you're using
#define DHTTYPE DHT11   // DHT 11 
//#define DHTTYPE DHT22   // DHT 22  (AM2302)
//#define DHTTYPE DHT21   // DHT 21 (AM2301)

// Initialize DHT sensor for normal 16mhz Arduino
DHT dht(DHTPIN, DHTTYPE);

// Create global varibales to store temperature and humidity
float t; // temperature in celcius
float f; // temperature in fahrenheit
float h; // humidity

// Configure software serial port
SoftwareSerial SIM900(7, 8);

// Create variable to store incoming SMS characters
char incomingChar;

void setup() {
  dht.begin();
  
  Serial.begin(19200); 
  SIM900.begin(19200);

  // Give time to your GSM shield log on to network
  delay(20000);
  Serial.print("SIM900 ready...");

  // AT command to set SIM900 to SMS mode
  SIM900.print("AT+CMGF=1\r"); 
  delay(100);
  // Set module to send SMS data to serial out upon receipt 
  SIM900.print("AT+CNMI=2,2,0,0,0\r");
  delay(100);
}

void loop(){
  if (SMSRequest()){
    if(readData()){
      delay(10);
      // REPLACE THE X's WITH THE RECIPIENT'S MOBILE NUMBER
      // USE INTERNATIONAL FORMAT CODE FOR MOBILE NUMBERS
      SIM900.println("AT + CMGS = \"+260975289856\"");
      delay(100);
      // REPLACE WITH YOUR OWN SMS MESSAGE CONTENT
      String dataMessage = ("Temperature: " + String(t) + "*C " + " Humidity: " + String(h) + "%");
      // Uncomment to change message with farenheit temperature
      // String dataMessage = ("Temperature: " + String(f) + "*F " + " Humidity: " + String(h) + "%");      
      
      // Send the SMS text message
      SIM900.print(dataMessage);
      delay(100);
      // End AT command with a ^Z, ASCII code 26
      SIM900.println((char)26); 
      delay(100);
      SIM900.println();
      // Give module time to send SMS
      delay(5000);  
    }
  }
  delay(10); 
}

boolean readData() {
  //Read humidity
  h = dht.readHumidity();
  // Read temperature as Celsius
  t = dht.readTemperature();
  // Read temperature as Fahrenheit
  f = dht.readTemperature(true);

  // Compute temperature values in Celcius
  t = dht.computeHeatIndex(t,h,false);

  // Uncomment to compute temperature values in Fahrenheit
  //f = dht.computeHeatIndex(f,h,false);
  
  // Check if any reads failed and exit early (to try again).
  if (isnan(h) || isnan(t) || isnan(f)) {
    Serial.println("Failed to read from DHT sensor!");
    return false;
  }
  Serial.print("Humidity: "); 
  Serial.print(h);
  Serial.print(" %\t");
  Serial.print("Temperature: "); 
  Serial.print(t);
  Serial.print(" *C ");
  //Uncomment to print temperature in Farenheit
  //Serial.print(f);
  //Serial.print(" *F\t");
  return true;
}

boolean SMSRequest() {
  if(SIM900.available() >0) {
    incomingChar=SIM900.read();
    if(incomingChar=='S') {
      delay(10);
      Serial.print(incomingChar);
      incomingChar=SIM900.read();
      if(incomingChar =='T') {
        delay(10);
        Serial.print(incomingChar);
        incomingChar=SIM900.read();
        if(incomingChar=='A') {
          delay(10);
          Serial.print(incomingChar);
          incomingChar=SIM900.read();
          if(incomingChar=='T') {
            delay(10);
            Serial.print(incomingChar);
            incomingChar=SIM900.read();
            if(incomingChar=='E') {
              delay(10);
              Serial.print(incomingChar);
              Serial.print("...Request Received \n");
              return true;
            }
          }
        }
      }
    }
  }
  return false;
}