Don't mention the web site in the opening prompt

Chances are that the people calling you not only already know that you have a web site, but that they got hold of your phone number from the web site. Unless this is not the case (your users are answering an infomercial, for instance), don't risk insulting your users' intelligence or making them feel that you really don’t want to interact with them over the phone. If you have to mention the web site, try to mention a specific page where they can find help, and mention it at moments where you have determined that the help they are seeking could be obtained via the web. A good place to mention the web site is at the closing of the call.