Not receiving the complete messange sent by SIM900
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);
  }
}