Use Pushover and a webhook from Uptime Robot to send notifications to your phone

Pushover+UptimeRobot.php

<?php
require __DIR__ . '/vendor/autoload.php';
use Sly\PushOver\Model\Push;
use Sly\PushOver\PushManager;

if (!isset($_GET['key']) || $_GET['key'] !== 'KEY') {
    die('No key');
}

$monitorFriendlyName = $_GET['monitorFriendlyName'];
$alertTypeFriendlyName = $_GET['alertTypeFriendlyName'];
$alertDetails = isset($_GET['alertDetails']) ? $_GET['alertDetails'] : null;

/**
 * First, create your own push, with its message.
 */
$myPush = new Push();

$message = "$monitorFriendlyName: $alertTypeFriendlyName";

if ($alertDetails) {
    $message .= " — $alertDetails";
}

$myPush->setMessage($message);
$myPush->setTitle('Uptime Robot'); // Optional

/**
 * Create an instance for PushManager.
 * Give it your user key (first arguement) and token one (second argument).
 * You can give a device name on third argument (optional).
 */
$pushManager = new PushManager('', '');

/**
 * Push it! :)
 */
if ($mySentPushInformations = $pushManager->push($myPush)) {
    /**
     * Your message has been sent.
     * $mySentPushInformations contents your sent push informations.
     */
}

//monitorID=*monitorID*&monitorURL=*monitorURL*&monitorFriendlyName=*monitorFriendlyName*&alertType=*alertType*&alertTypeFriendlyName=*alertTypeFriendlyName*&alertDetails=*alertDetails*&monitorAlertContacts=*monitorAlertContacts*
?>