um exemplo de código para envio de SMS, o codigo pode ser melhorado e optimizado em muitos pontos.

enviando_sms.ino

#include <SoftwareSerial.h>

SoftwareSerial mySerial(2, 3); //RX, TX


//Ligando o Shield GSM SIM900.
int powerkey =  5;
int statuspin = 6;
int pinState = 0;

//Definindo Botao:
const int buttonPin = 13;
const int buttonPin2 = 12;
const int buttonPin3 = 11;
const int buttonPin4 = 10;
const int buttonPin5 = 9;

//Variavel de Apoio:
bool buttonState = LOW;
bool buttonState2 = LOW;
bool buttonState3 = LOW;
bool buttonState4 = LOW;
bool buttonState5 = LOW;

void setup()
{
  pinMode(buttonPin, INPUT);
  pinMode(buttonPin2, INPUT);
  pinMode(buttonPin3, INPUT);
  pinMode(buttonPin4, INPUT);
  pinMode(buttonPin5, INPUT);

  pinMode(powerkey, OUTPUT);
  pinMode(statuspin, INPUT);

  //Serial Por Software Para o SIM900 Funcionar.
  mySerial.begin(9600);

  Serial.begin(9600);
  //Delay Necessario Para o SIM900 Estabilizar.

  delay(10000);

}

void loop()
{
  pinState = digitalRead(statuspin);
  if (pinState == LOW) {
    digitalWrite(powerkey, HIGH);
    delay(2000);
    digitalWrite(powerkey, LOW);
  }

  buttonState = digitalRead(buttonPin);
  if (buttonState == HIGH)
  {
    delay(20000);
    mySerial.println("AT+CMGF=1");
    delay(200);
    mySerial.print("AT+CMGS=");
    mySerial.write(byte(34));
    mySerial.print("11974389125");
    mySerial.write(byte(34));
    mySerial.println();
    delay(200);
    mySerial.print("A porta do motorista foi aberta.");
    mySerial.write(byte(26));
    while (1);
  }

  buttonState2 = digitalRead(buttonPin2);
  if (buttonState2 == HIGH)
  {
    delay(20000);
    mySerial.println("AT+CMGF=1");
    delay(200);
    mySerial.print("AT+CMGS=");
    mySerial.write(byte(34));
    mySerial.print("11974389125");
    mySerial.write(byte(34));
    mySerial.println();
    delay(200);
    mySerial.print("A porta do passageiro foi aberta.");
    mySerial.write(byte(26));
    while (1);
  }

  buttonState3 = digitalRead(buttonPin3);
  if (buttonState3 == HIGH)
  {
    delay(20000);
    mySerial.println("AT+CMGF=1");
    delay(200);
    mySerial.print("AT+CMGS=");
    mySerial.write(byte(34));
    mySerial.print("11974389125");
    mySerial.write(byte(34));
    mySerial.println();
    delay(200);
    mySerial.print("A porta de tras do motorista foi aberta.");
    mySerial.write(byte(26));
    while (1);
  }

  buttonState4 = digitalRead(buttonPin4);
  if (buttonState4 == HIGH)
  {
    delay(20000);
    mySerial.println("AT+CMGF=1");
    delay(200);
    mySerial.print("AT+CMGS=");
    mySerial.write(byte(34));
    mySerial.print("11974389125");
    mySerial.write(byte(34));
    mySerial.println();
    delay(200);
    mySerial.print("A de tras porta do passageiro foi aberta.");
    mySerial.write(byte(26));
    while (1);
  }
  
  buttonState5 = digitalRead(buttonPin5);
  if (buttonState5 == HIGH)
  {
    delay(20000);
    mySerial.println("AT+CMGF=1");
    delay(200);
    mySerial.print("AT+CMGS=");
    mySerial.write(byte(34));
    mySerial.print("11974389125");
    mySerial.write(byte(34));
    mySerial.println();
    delay(200);
    mySerial.print("O porta malas foi aberto.");
    mySerial.write(byte(26));
    while (1);
  }
}