How do I forward incoming calls to voice-mail on my office phone?

On my office phone, an inter-tel 3000, there used to be a feature allowing me to forward a call to voice-mail as the extension was ringing. However, after resetting the system, this option no longer appears. I've searched through all kinds of manuals, doentations, and websites without finding a solution. I think I have to do something with the "system programming" feature, but there are so many options that make no sense to me. What should I do to get that little "forward to voice-mail" option back?