I read up on InformationWeek there’s a new service out there called SlyDial. There is no web site for this; it’s just a phone number. What happens is that if you want to call someone but don’t want to talk to them and go directly to voicemail, SlyDial is it. Just punch in (276)SLY-DIAL and let the service do the dirty work for you. No it’s not free but cheap enough (15 cents a call).
So if you’re the non-confrontational type (or just don’t feel like talking to certain people), SlyDial will let you skip the conversation and let you go straight to voicemail – just like you wanted to.
[Source: InformationWeek]

I thought you could already do this by calling your own voicemail and choosing the option to leave a voicemail for someone else.
After trying it I realized that it will only work on your own carrier. For instance since I have T-mobile I can leave other T-mobile users voicemail without calling. Guess there is a use for this although I don’t know what it would be.