You can do the following:
$fp = fopen('http://servertocheck.com','r');
mail('email@example.com', 'Server Down yayyyy', 'hey wake up your server\'s down');
You can find your cellphone's email-to-sms address by just looking around on google. For Rogers (Canadian Provider), its pcs.rogers.com. Say my number was 416-123-1234, my email would be firstname.lastname@example.org
Make sure it's on a different server from the one you're checking.. else you won't get the message obviously.
You can stick this snippit in some global php file on a fairly large site so once someone accesses that site, they're involuntarly also checking your site for uptime. =) Or you can stick this in a cronjob using curl.