Not receiving the complete messange sent by SIM900 I need help with SIM900 and Arduino Mega. I do not receive the complete message that I send from my phone. (Arduino Mega _ SIM900 _ Alimentation 9V/2A) 

Message sent:

1234567891234567890


Message received:

12345678912


String Char;
void setup() {
  Serial.begin(9600);
  Serial2.begin(9600);

}

void loop() {
  while (Serial2.available()) {
    Char += (char)Serial2.read();
  }
  Serial.println(Char);
  delay(1000);
}
change the test sketch to

void setup() {
  Serial.begin(9600);
  Serial2.begin(9600);
}

void loop() {
  if (Serial2.available()) {
    char c = Serial2.read();
    Serial.write(c);
  }
}


or use the SerialPassthrough sketch from examples

or

void setup() {
  Serial.begin(9600);
  Serial2.begin(9600);
  Serial2.setTimeout(100);
}

void loop() {
  if (Serial2.available()) {
    String s = Serial2.readString();
    Serial.println(s);
  }
}