When problems arise, anyone wants solutions, including your customer. Most of your customers are not unreasonable sort of people. They understand that problems appear and mistakes do happen. But, of course, they want to promptly communicate any problems that arise around them, especially during emergencies.
One thing to keep in mind is that the web is all about communication. Web hosts should really understand this and put measures into place that allows for the most reliable and uninterrupted communication with their customers.

Actually, there isn't a web host out there who hasn't experienced times where the server that their company web site is on has gone down; sometimes only for a few minutes, but sometimes those minutes can turn into hours, or even days, in spite of the reasons. And in most cases, you don't dedicate an entire server to your domain – you have customers who also share that server. How can they find out what is going on with their website, and more importantly, when it will be back online? Make sure that your customers can reach you by that time.

A recent survey shows that over 75% of web hosting companies do NOT publish a technical support number. Yet, some hosts have emergency support available by telephone. In that case, if they can remember the phone number since they cannot access your website to find the number, they begin calling, and calling, and calling. Usually there are over 100-200 customers hosted on a single server. Do you or your staff really have the time to be fielding that many phone calls when you are in the middle of a crisis and need to devote your time to fixing the problem and getting the server operational? Probably not. Nor do your customers want to spend time on hold or dealing with busy signals. Every minute on hold, or every call that results in a busy signal, leaves a bad impression with that customer. They want to know WHY they are down and they want to know NOW.

If you are large enough to have multiple incoming lines and an automated voice mail system, a simple solution is to immediately record a server status alert, along with an honest assessment of what has happened and, if you know it, an estimated time that the server will be operational again. This will allow customers calling about their site being down to get the information they need without having to talk to a live technician. The customer is happy to be informed, even if they are VERY unhappy about having their site down, and you have saved yourself time and money and can focus on the important problem at hand. In your recorded status alert, let the customer know how often the status will be updated. It would be recommended that if a server is down longer than 90 minutes, updated information should be made available every 30-45 minutes.
If you are a smaller host with only one incoming line, you probably don't have enough customers that busy signals will become a major irritant, but keep the message very short and to the point.