Don't know if this is what you're talking about, but it's what I did:
Put an entry in your phone book with any name and the number being the
"p" (pause) key. Then, make the ringtone for that entry be silent.
Technically, you can answer it if you happen to be looking at the phone
when the call comes in, but if the phone is on a clip or in your
pocket/pocketbook, you won't even know someone called and voice mail will
pick it up.