To transfer a caller directly to voicemail you'll need to create a new Voicemail Profile and CTI route point.
For the purpose of this discussion we will say your dialing plan has the leading digit of "5" set aside for DIDs and you have four-digit extensions. Also, we will be using the "*" key to identify a VM transfer.
1. To create the Voicemail Profile go to Feature -> Voice Mail -> Voice Mail Profile and then click on "Add a New Voice Mail Profile". You will need to fill in the following:
a. Voicemail Profile Name: Make the profile name something distinctive. I used TransferToVM.
b. Description: Describe it.
c. Voicemail Pilot: Choose your voicemail pilot number from the drop down list.
d. Voice Mail Box Mask: If you have 4-digit extensions use "XXXX" - drop the quotes but use the X's.
2. Next, you'll need to create a route point by going to Device -> CTI Route Point and then clicking on "Add a New CTI Route Point". You will need to fill in the following fields:
a. Device Name: I try to make this distinctive so I know that I'm looking at VM-related route points. For me, xfer to VM_(location code) works well because I know that it will be at the end of my search list if I look at all CTI route points at once.
b. Description: Not necessary but provides more detail for the next guy.
c. Device Pool, Calling Search Space, Location, etc.: I think you know what to do here...
d. Click on the Insert button to add the new route point
3. Now you need to add the DN range(s) for extensions that you want to route to VM. To do so click on Line 1 - Add DN.
Above I stated that this scenario uses 4-digit extensions beginning with "5". In order to route all numbers beginning with "5" to voicemail, the DN would be "*5XXX" - don't use the quotes but do put in the * and the X's. So...
a. Number Field: *5XXX
b. Partition: Select the correct internal partition
c. Voice Mail Profile: Use the profile you created in step 1a.
d. Calling Search Space: Choose the correct internal CSS.
e. Forward All: Check the Voice Mail check box and then put in the CSS from 4d.
f. You may want to change the Multiple Calls/Call Waiting Settings section to accommodate more than the standard number of calls allowed.
g. Click Update Device.
If a field wasn't mentioned, it was left at the default value.
Now, pick up your phone and dial "*" plus your extension. You should hear your voicemail greeting.