...It's old technology.  When it breaks, it's hard to troubleshoot (hint - it's the hardware).  When it works perfectly, nobody thanks you.  If it's not set up correctly, the entire office will let you know.  That's why you need this guide:

Step-by-Step Guide for Configuring and Using Fax on Windows Small Business Server 2003

http://www.microsoft.com/downloads/details.aspx?FamilyID=1b7510a0-1595-4776-a101-c3bec7512594&displaylang=en

The document is what it says it is - a step-by-step walkthrough for setting up and configuring fax on your SBS server.