Send an SMS on an Arduino Uno

// Send an SMS: the SIM800L sketch every tutorial starts with.
//
//   SIM800L TXD  -> pin 3   (the sketch's RX)
//   SIM800L RXD  <- pin 2   (the sketch's TX)
//   SIM800L VCC  -> an 18650 cell's +, GND -> the cell's - and the Uno's GND
//
// The modem wants 3.4 to 4.4 V and 2 A peaks, so it has a Li-ion cell of its
// own. On the Uno's 5 V pin it switches itself off and back on, over and over,
// as the real one does.
//
// The Uno's one hardware serial port belongs to the serial monitor, so the
// modem gets a SoftwareSerial port at 9600 baud. A bare AT lets the modem's
// autobaud find that rate; then text mode, the number, the text, and Ctrl+Z
// (character 26) to send it.
//
// There is no real phone network here. The modem has its own line in the
// serial monitor, and that line is the simulated network: the SMS shows up
// there about three seconds after Ctrl+Z. Type sms hello in it to text the
// modem back, and the sketch prints what the modem says.

#include <SoftwareSerial.h>

SoftwareSerial mySerial(3, 2); // RX, TX: the SIM800L'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();
  mySerial.println("AT+CMGS=\"+15551234567\""); // the number to text
  updateSerial();
  mySerial.print("Hello from Mokxi"); // the message
  updateSerial();
  mySerial.write(26); // Ctrl+Z sends it
}

void loop() {
  updateSerial();
}