Arduino Mega 2560 with SIM808 module - Incomplete SMS response I have this code to read a SMS: 

String read_SMS(int index)
{
  Serial.println("--START READ SMS POSITION" + String(index) + "--");
  delay(400);
  sendATcommand("AT+CMGF=1", "OK", 1000);    // sets the SMS mode to text
  delay(2000);
  String tmp;
  tmp = readATcommand("AT+CMGR=1", 4000);
  Serial.println("Content: " + tmp);
  Serial.println("--END READ SMS POSITION" + String(index) + "--");
}

String readATcommand(String ATcommand, unsigned int timeout)
{
  char response[800];
  unsigned long previous;
  memset(response, '\0', 800);    // Initialice the string

  int x = 0;

  GSMSerial.println(ATcommand);
  delay(3000);
  previous = millis();
  do {
    if (GSMSerial.available() &gt; 0) {
      response[x] = GSMSerial.read();
      x++;
    }
  } while ((millis() - previous) &lt; timeout);   // Waits for the asnwer with time out

  return (String) response;
}

// sending AT commands to the Sim808 module
int8_t sendATcommand(String ATcommand, char* expected_answer, unsigned int timeout) {

  uint8_t x = 0, answer = 0;
  char response[200];
  unsigned long previous;

  memset(response, '\0', 200);    // Initialice the string

  delay(100);

  // Clean the input buffer
  while (GSMSerial.available() &gt; 0 ) GSMSerial.read();

  if (ATcommand[0] != '\0') {
    // Send the AT command
    GSMSerial.println(ATcommand);
    //Serial.print(F("AT Command: "));
    //Serial.println(ATcommand);
  }

  // Waiting for the answer
  x = 0;
  previous = millis();

  do {

    // if there are data in the UART input buffer, reads it and checks for the asnwer
    if (GSMSerial.available() &gt; 0) {

      response[x] = GSMSerial.read();
      x++;
      // check if the desired answer (OK) is in the response of the module
      if (strstr(response, expected_answer) != NULL) {
        answer = 1;
      }
    }

  } while ((answer == 0) &amp;&amp; ((millis() - previous) &lt; timeout));   // Waits for the asnwer with time out

  //Serial.println(F("START RESPONSE:"));
  //Serial.println(response);
  //Serial.println(F("END RESPONSE"));
  //if (answer == 0) {
  //  Serial.println("Could not get the expected answer!");
  //}
  //else {
  //  Serial.println("Found expected answer!");
  //}

  return answer;
}


What can be wrong in the readATcommand function as it replies with an incomplete response: 

--START READ SMS POSITION1--
Content: 
AT+CMGR=1

+CMGR: "REC READ","+407245402XX","","17/06/29,10:
--END READ SMS POSITION1--


But, when I run it from a terminal, the AT+CMGR=1 replies with: 

AT+CMGR=1

+CMGR: "REC READ","+40724540234","","17/06/29,10:03:06+12"
Salut

OK


As you can see the message body is not there + the time of the SMS is also incomplete.

Thank you!
Don't delay.

In the three seconds that you wait between sending the AT command and reading the response the entire response has arrived and been placed in the serial RX buffer - which has overflowed because there's more than 64 bytes in the response. So you lose data.

You should never use delay() when reading from serial, it's dangerous (as you see).

Instead you should be just immediately reading until you see the correct response from the command you sent - which will either be "OK" or "ERROR". Everything before that is the content of the response to your command.