In the days of yore, before the Internet, there were pretty much only two ways for a business to communicate with their customers -- postal service mail or by telephone. With the advent of the Internet and the World Wide Web, this is no longer the case. Besides the obvious postal service mail and telephone contact methods, there are now numerous other ways you can communicate with your customers for help desk issues.
One way would be via email. Just about everyone has an email address, and with all of the free email providers out there (Google GMail, Microsoft Hotmail, Yahoo Mail, etc.) most people are able to have an email address they can use. Most decent help desk Software packages will provide you with a means of communicating with your customers requiring help desk assistance via email. Email is easy to use, easy to access, and provides a way to quickly send messages.
Another way would be though Instant Messaging (IM). Just like email, IM software abounds (AoL Instant Messenger, MSN Messenger, Yahoo Messenger, etc.). Likewise, many help desk Software packages provide an internal instant messaging capability where you can host your own instant messaging service and have customers contact you via this service rather than via a public service. Benefits to using IM are that they're generally free, provide instant feedback, and give the customer a better help desk experience.
Remember that customer's don't like to be "brushed off" when working with help desk personnel. Therefore, you need to ensure you have policies and procedures in place that keep the customer from feeling that the help desk person left them before the problem was resolved to the customer's full satisfaction.
Finally, a really good way for a help desk person to work with and communicate with a help desk customer is via some type of remote access. The later versions of Microsoft Windows offer what is called "Remote Assistance". Users can allow your support professionals to access the user's machines and perform actions for the user to solve issues.
You can also have your support professionals use Microsoft's Netmeeting to "dial-up" a remote computer and share desktops to demonstrate how to solve a problem or get additional information from the remote desktop to resolve an issue.
There are also a number of additional third-party solutions you can use to assist you in remote support. One such is WebEx. Here is a list of other web conferencing tools you may wish to consider:
As you can see, there are a lot of options available for you in contacting your customers for help desk support. You may need to evaluate your needs against the methods available to find the right solutions for your help desk contacts.