I am using V15 PRO. I set up custom speeddial buttons on the windows client using DVM Username and *4ext. If I need to send a caller to voice mail, I only have to click the DVM Username speeddial button and it transfers.
You can right-click on the button you plan to use and configure it under PRO. On the windows client, you can right-click on the screen number at the bottom to add up to 3 BLF screens, which give you 108 buttons. Beyond this, you would have to do it from the 3CX console.
I set mine up as the odd number being the normal BLF and the next even number being the direct to voicemail button. This makes it easier to find and takes about 5 minutes to train someone to use it.