Use Google Apps Script to monitors website domains and send SMS text alerts when any of these sites are down or inaccessible.

function init() {
  if (ScriptApp.getScriptTriggers().length == 0) {
    // Set up a monitor that triggers every 5 minutes
    ScriptApp.newTrigger('websiteMonitor').timeBased().everyMinutes(5).create();
  }
}

function websiteMonitor() {
  var response, error, code, urls;

  // The script can monitor multiple website URLs (comma separated)
  urls = SpreadsheetApp.getActiveSheet().getRange('B2').getValue();
  urls = urls.replace(/\s/g, '').split(',');

  for (var i = 0; i < urls.length; i++) {
    var url = urls[i];

    if (!ScriptProperties.getProperty(url)) {
      ScriptProperties.setProperty(url, 200);
    }

    // Trying to connect to the website URL
    try {
      response = UrlFetchApp.fetch(url);
    } catch (error) {
      // If URLFetchApp fails, the site is probably down
      updateLog(url, -1);
      continue;
    }

    code = response.getResponseCode();
    updateLog(url, code);
  }
}

function updateLog(url, code) {
  if (ScriptProperties.getProperty(url) == code) return;

  ScriptProperties.setProperty(url, code);

  var sheet = SpreadsheetApp.getActiveSheet();

  var row = sheet.getLastRow() + 1;
  var time = new Date();
  var msg = 'Down';

  if (code == 200) msg = 'Up';

  msg = 'Website is ' + msg + '  ' + url;

  sheet.getRange(row, 1).setValue(time);
  sheet.getRange(row, 2).setValue(msg);

  // Send an email notification when the site status changes
  var email = sheet.getRange('B3').getValue();
  MailApp.sendEmail(email, msg, url);

  var now = new Date(time.getTime() + 10000);

  // Create an event in Google Calendar with an SMS reminder
  if (sheet.getRange('B4').getValue().toLowerCase() == 'yes') CalendarApp.createEvent(msg, now, now).addSmsReminder(0);
}