#include <SoftwareSerial.h>

SoftwareSerial sim(8, 9); // RX, TX

bool firstSIMCommand = true;

String sendSIMCommand(String cmd) {
  sim.println(cmd);
  delay(100);
  auto start = millis();
  while (!sim.available()) {
    auto now = millis();
    if (now - start >= 500) {
      Serial.println("(SIM timeout)");
      return "";
    }
  }
  if (firstSIMCommand) { // TODO: It removes \0 because it do not allows use indexOf() But why this \0 appears here?
    sim.read();
    firstSIMCommand = false;
  }
  auto res = sim.readString();
  auto newLine = res.indexOf("\n");
  res.remove(0, newLine + 1);
  res.trim();
  return res;
}

void setup() {
  Serial.begin(9600);
  while (!Serial) ; // Arduino Pro Micro / Leonardo
  Serial.println("Start!");
  sim.begin(9600);

  auto res = sendSIMCommand("AT");
  if (res == "OK") {
    Serial.println("SIM check OK");
  } else {
    Serial.println("SIM check failed, response for AT:");
    Serial.println(res);
  }

  res = sendSIMCommand("AT+CBC");
  Serial.println(res);

  res = sendSIMCommand("AT+COPS?");
  Serial.println(res);
}

void loop() {
  if (sim.available())
    Serial.write(sim.read());
  if (Serial.available())
    sim.write(Serial.read());
}

  while (!Serial) ; // Arduino Pro Micro / Leonardo