void send_sms(char* text){
  Serial.println("AT+CMGF=0"); // Set PDU mode
  delay(3000);

  String phone_output = "0011000B91";

  String phone_number = calculate_phone_number();
  phone_output.concat(phone_number);

  phone_output.concat("0000AA");  // TP-PID, TP-DCS, TP-validity

  int msg_len = strlen(text);
  String hex_len = pad_hex(String(msg_len, HEX));

  Serial.println(hex_len);
  phone_output.concat(hex_len);
  String converted_text = convert_text_to_pdu(text);

  phone_output.concat(converted_text);

  int number_of_octets = phone_output.length() / 2;
  Serial1.print("AT+CMGS=");
  Serial1.println(number_of_octets);
  delay(3000);

  Serial1.print(phone_output);
  delay(3000);

  Serial1.write(26);
  delay(3000);

  Serial.println("sent!");
}

String calculate_phone_number(){
  return "8346355555F5";
}

String convert_text_to_pdu(char* text){
  String converted = "";

  int data_length = strlen(text);
  int to_append_index = 1;
  for (int x = 0; x < data_length; x++){
    String curr_char = String(text[x], BIN);

    if(to_append_index == 8){
      to_append_index = 1;
      continue;
    }

    curr_char = String(curr_char.substring(0, curr_char.length() - (to_append_index - 1)));

    String to_append = "";
    if(x != data_length - 1){
      // not last character
      String next_char = String(text[x+1], BIN);
      to_append = String(next_char.substring(next_char.length() - to_append_index));
    }
    curr_char = to_append + curr_char;

    int converted_len = curr_char.length();
    int convert_char = 0;
    for(int bi = 0;bi<converted_len;bi++){
      char c = curr_char.charAt(bi);
      int curr_bit = c-'0';  // convert to int

      int partial = curr_bit * 1 << (converted_len-1-bi);
      convert_char += partial;
    }
    String hex_char = pad_hex(String(convert_char, HEX));

    converted.concat(hex_char);  // concatenate to return string

    to_append_index++;
  }

  return converted;
}

String pad_hex(String hex){
  if(hex.length() % 2 != 0){
    return 0 + hex;
  }
  return hex;
}