Help!!reading received sms truncated PDU format

Hi everyone i am facing some problems in my project. My goal is to turn on the LED in pin 13, By sending a text "on" to my arduino, but when i read the message in PDU format. the recieved PDU message is truncated or incomplete. I have tried adding more delays, but it doesnt solve the problem. Please help me..

 int indicatorPin=13; //Indicator LED pin 13 for test with led.
 String readString;

 char c;

 void delsms(){
    Serial.println("AT+CMGD=1");//Delete sms possition 1
    }
 void readsmsAT(){
   Serial.println("AT+CMGR=1");//Read sms possition 1 AT command
   delay(3000);
   }


   void setup() {
         
  pinMode(indicatorPin,OUTPUT);
  delay(12000);//first time connect cable arduino with phone...
  Serial.begin(9600);
  delay(3000);
  Serial.println("AT+CPMS=\"ME\",\"SM\"");//Set memory phone for store  sms to SIM
  delay(3000);
  Serial.println("ATE=0");//Echo off
  delay(3000);

}

   void loop()
  {
       readString=0;//format readString...
       readsmsAT();//read sms repeatedly
     
      // read phone reply if available
    while(Serial.available() > 0)
            {
                
                Serial.print( c=Serial.read());
               
               
                readString+=c;
            }
               delay(1000);
               
               Serial.println(readString);//serial monitor display sms pdu format
               
               delay(3000);
              if ( readString.indexOf("26F37")>0)//  pdu on is 26F37
              {
                 digitalWrite(indicatorPin, HIGH);//only for test led on
                 
                 delay(5000);
                 
                 digitalWrite(indicatorPin, LOW);//only for test
              }
              
              if (readString.length()>40)//delete sms if sms received
              {
              delsms();//delete sms
          }
              delay(10000);//delay start loop again for check new sms...

  }

Need some help over here!

The problem is Serial buffer size, by default it is only 16B. You need to increase it in the Arduino folder "\hardware\arduino\cores\arduino\HardwareSerial.cpp" file

The problem is Serial buffer size, by default it is only 16B

But only for the most RAM-limited processors. Normally, it is 64.

I have exactly the same problem, is there any solution?

First question. Any particular reason that you're using PDU mode? Text mode, and live SMS notifications (CNMI & CMT messages) would make your job a lot easier!

Second observation. Using delay() calls, and 'String' variables will make life much harder than it needs to be.

Third question. What processor are you using? Relevant to the RAM requirements. Worst case per message received in TEXT mode, is about 60 chars of date, time, sender... PLUS the actual message body... This can be reduced if you clean up as you go.