Receive an SMS on an Arduino Uno

// Receive an SMS: the "receive" sketch the GSM tutorials print, here on a
// SIM900A board.
//
//   SIM900A TXD  -> pin 3   (the sketch's RX)
//   SIM900A RXD  <- pin 2   (the sketch's TX)
//   SIM900A VCC  -> a 5 V 2 A adapter, GND -> the adapter's - and the Uno's GND
//
// The SIM900A board's regulator takes 5 V, but the modem draws 2 A when it
// transmits, more than the Uno's 5 V pin can give: it has an adapter of its own.
//
// AT lets the modem's autobaud find 9600 baud, AT+CMGF=1 picks text mode, and
// AT+CNMI=1,2,0,0,0 asks for every new message to be printed as it arrives,
// as +CMT with the number and the time, then the text.
//
// There is no real phone network here. The modem has its own line in the
// serial monitor, and that line is the simulated network: type sms hello in it
// and the Uno prints the message. Type AT commands in the Uno's line to talk
// to the modem yourself.

#include <SoftwareSerial.h>

SoftwareSerial mySerial(3, 2); // RX, TX: the SIM900A's TXD and RXD

void updateSerial() {
  delay(500);
  while (Serial.available()) {
    mySerial.write(Serial.read()); // the serial monitor to the modem
  }
  while (mySerial.available()) {
    Serial.write(mySerial.read()); // the modem to the serial monitor
  }
}

void setup() {
  Serial.begin(9600);
  mySerial.begin(9600);
  Serial.println("Initializing...");
  delay(1000);

  mySerial.println("AT");                // the handshake: the modem answers OK
  updateSerial();
  mySerial.println("AT+CMGF=1");         // text mode
  updateSerial();
  mySerial.println("AT+CNMI=1,2,0,0,0"); // print new messages as they come
  updateSerial();
}

void loop() {
  updateSerial();
}