How to create variable from sd card in arduino

I have saved a number onto a sd card in a file called CONFIG.BAT, it is a telephone number and is in the format of "+441234567890", in the serial monitor I can print it out using

      myFile = SD.open("CONFIG.DAT");
      
      if (myFile) {
        
    Serial.println("CONFIG.DAT:");
    
    // read from the file until there's nothing else in it:
    while (myFile.available()) {
     
          Serial.write(myFile.read());
    
    }

And I get this responce

    Initializing SD card...initialization done.
    CONFIG.DAT:
    +441234567890

What I want to do is put this number into a variable "telNo", I have tried to use

    telNo = (myFile.read());

The response I get is

    Initializing SD card...initialization done.
    CONFIG.DAT: 
    10

I'm not sure what this "10" signifies.

I have also tried "concat"

    while (myFile.available()) {

            number.concat(myFile.read());      

    }

The response I get is the Decimal code for the number with some numbers on the end I don't understand

    Initializing SD card...initialization done.
    CONFIG.DAT: 
    48555653515353555354561310

The last four numbers (1310) I don't understand

What I'm looking to achieve is "telNo = the telephone number in the CONFIG.DAT file, so if any one could help I would appreciate it

What I want to do is put this number into a variable “telNo”, I have tried to use

The read() method returns one byte. Whether that is appropriate to store in telNo, or not, depends on what type telNo is. Whether that single line means anything, or not, depends on the rest of the code you didn’t post.

The response I get is the Decimal code for the number with some numbers on the end I don’t understand

The problem is… Well, that’s a snippet of the answer. If you want me to post ALL of the answer, well, I’m guessing you can figure out what you need to do.

I didn't post all of the code as I didnt want to get complaints of too much code, basically I want to assign the telephone number held in the CONFIG.BAT file of the sd card to the "telNo" which I can call within the AT command to send a sms.

I totally missed your point in this reply

"The problem is... Well, that's a snippet of the answer. If you want me to post ALL of the answer, well, I'm guessing you can figure out what you need to do."

If I could figure it out I wouldn't be asking and I have been trying to resolve this for hours now

Here is all of my sketch

#include <SoftwareSerial.h>
//#include <SD.h>
#define SD_CS_PIN SS
#include <SPI.h>
#include "SdFat.h"

SdFat SD;

File myFile;

//SIM800 TX is connected to Arduino D8
#define SIM800_TX_PIN 8
 
//SIM800 RX is connected to Arduino D7
#define SIM800_RX_PIN 7
 
//Create software serial object to communicate with SIM800
SoftwareSerial serialSIM800(SIM800_TX_PIN,SIM800_RX_PIN);


//const int ledPin3 = 11;//Define the Bluetooth led pin
const int ledPin2 = 2;//Define the interupt pin to signify bluetooth connect or disconnect

String readString = "";
String telNoString = "";
String number = "";

volatile int ledonState = 0;
int lastLedonState = 0;


const int thresholdvalue=680;//The threshold to turn the led on


void setup() {
  pinMode (ledPin2, INPUT);//set input for interupt pin
  //pinMode (ledPin3, OUTPUT);//set output for bluetooth pin

  attachInterrupt(0, pin_ISR, CHANGE);
  
  Serial.begin(9600);

  while(!Serial);
   
  //Being serial communication witj Arduino and SIM800
  serialSIM800.begin(9600);
  delay(1000);

  SysCall::yield();
    
  Serial.println("Setup Complete!");

}


void pin_ISR() {
  ledonState = digitalRead(ledPin2);

if (ledonState != lastLedonState) {

  if (ledonState == HIGH) {
        //digitalWrite(ledPin3, HIGH);//turn led on
            Serial.println("HC-05 is now connected");
            //Serial.println();
      }else{
         //digitalWrite(ledPin3, LOW);//turn led off
             Serial.println("HC-05 is now Disconnected");
             //Serial.println();
  }
lastLedonState = ledonState;
}
}


void sound_detect(){
  
  int sensorValue = analogRead(A0);//use A0 to read the electrical signal
  
  if(sensorValue > thresholdvalue) {
    
//  digitalWrite(ledPin1,HIGH);//if the value read from A0 is larger than 400,then light the LED
//  delay(10);
//  digitalWrite(ledPin1,LOW);

 Serial.print("Initializing SD card...");
  
  pinMode(10, OUTPUT);

  if (!SD.begin(SD_CS_PIN)) {
    Serial.println("initialization failed!");
    return;
  }
  
  Serial.println("initialization done.");  

  if (!SD.exists("CONFIG.DAT")) {

    Serial.println("No Number Exists! Please go to Setup Device to add Number for Alert");
    
  }else{

  // open the file for reading:
  myFile = SD.open("CONFIG.DAT");
  
  if (myFile) {
    
    Serial.println("CONFIG.DAT:");

    // read from the file until there's nothing else in it:
    while (myFile.available()) {

            number.concat(myFile.read());      
          //number = (myFile.read());
          //Serial.write(myFile.read());

    }

      //Serial.println(myFile);    
    
    // close the file:
    myFile.close();
    
  } else {
    
    // if the file didn't open, print an error:
    Serial.println("error opening CONFIG.DAT");
    
  }
 
  }

  
//    //Set SMS format to ASCII
//  serialSIM800.write("AT+CMGF=1\r\n");
//  delay(1000);
// 
//  //Send new SMS command and message number
//  serialSIM800.write("AT+CMGS=\"+44"+number+"\"\r\n");
//  delay(1000);
//   
//  //Send SMS content
//  serialSIM800.write("TEST SMS NOISE DETECT");
//  delay(1000);
//   
//  //Send Ctrl+Z / ESC to denote SMS message is complete
//  serialSIM800.write((char)26);
//  delay(1000);
//     
//  Serial.println("SMS Sent!");
Serial.println("this is the phone number: ");
Serial.print(number);

}

}



void insertNo(){

        while(Serial.available()==0) { // Wait for User to Input Data  
  }

telNoString = Serial.readString();


  Serial.print("Initializing SD card...");
  
  pinMode(10, OUTPUT);

  if (!SD.begin(SD_CS_PIN)) {
    Serial.println("initialization failed!");
    return;
  }
  
  Serial.println("initialization done.");

  if (SD.exists("CONFIG.DAT")) {
    
  SD.remove("CONFIG.DAT");
  }

  if (!SD.exists("CONFIG.DAT")) {
    
  myFile = SD.open("CONFIG.DAT", FILE_WRITE);

  // if the file opened okay, write to it:
  if (myFile) {
    Serial.print("Writing to CONFIG.DAT...");
    
    myFile.println(telNoString);
    
    // close the file:
    myFile.close();
    
    Serial.println("done.");
    
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening CONFIG.DAT");
  } 

  // re-open the file for reading:
  myFile = SD.open("CONFIG.DAT");
  
  if (myFile) {
    
    Serial.println("CONFIG.DAT:");

    // read from the file until there's nothing else in it:
    while (myFile.available()) {
      
      Serial.write(myFile.read());
      
    }
    
    // close the file:
    myFile.close();
    
  } else {
    
    // if the file didn't open, print an error:
    Serial.println("error opening CONFIG.DAT");
    
  }     

  }
   return;    
}


void loop() {

 while (Serial.available()) {
          delay(10);       
          char c = Serial.read();
          readString  += c;
 }
 
    if(readString == "setup device") {

      Serial.println(readString);
      readString = "";



insertNo();
      
    }else{
      if(readString == "start device") {
        
      Serial.println(readString);
      readString = "";


    }
 sound_detect();   
    }       
}

I have managed to find a resolution to this by using the following code

    while (myFile.available()) {

                char ltr = myFile.read();
                number += ltr;      
    }

now whenever I call "number" I get the telephone number held on the sd card

now whenever I call "number"

That would be never, because "number" isn't function, so you can't call it.