I wouldn't trust anything that doesn't monitor every minute. Those 'free' monitors only pull every 5. That's not terribly sufficient for a professional host. I wouldn't trust someone monitoring from a single (or random) location, either. You should consistently have 5 monitors running, at the very minimum.
phpservermonitor.org might work for you, if you can set it up to work with some sort of multiple host situation, where multiple check servers are used.
Tom Whiting, WHMCS Guru extraordinaire
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