WHMCS is great, but really it's not a server monitoring program. While it does check for ports on a server, there's much more to monitoring the server than what WHMCS can do.
Check out Nagios. This will allow you to not only get alerts on outages, but also monitor scripts on the server, check disk spaces, CPU usage, Memory and if you program on it, you can even get it to restart certain services based on conditions.
I've used many programs over the years, and even quite a few PAID software programs, but Nagios still remains one of the Top Dogs when it comes to server monitoring. And since it's open source, it's an even sweeter deal.