How would I got about getting an automated phone call if the server is not available?
Out web host said they don't offer anything like this. I think I remember hearing that Cacti can do this. However, is there a lighter weight solution?
Many system/application monitoring software offer this feature. Using your Cell provider's email -> sms service you can setup something like Nagios to alert you to issues. But I wouldn't consider any of the software 'lightweight' when compared to cacti which itself is fairly lightweight itself.