I've been testing out the new free service from Slydial that lets you leave a voicemail without ringing the other guy's phone.  I called 1-267-SLYDIAL (267-759-3425), entered the phone number I wanted to dial, and sure enough I was soon talking to directly to the voicemail box.

The service is free, although you do have to listen to a very short advertisement (for McDonalds dollar menu, when I called), but there are a few catches:

• It only works for U.S. mobile numbers.
• Doesn't work for non-mobile numbers (like, say, Microsoft's corporate number)

I also noticed that it worked from my AT&T mobile number, as well as from the landline I use at home, but for some reason it didn't work from my VOIP account.  I'm contacting my VOIP provider to see if they know why.

[update: it was a fluke.  I tried again and it works fine with my VOIP account]