I need to write a How-To on this.
The best way is to create a "phantom" extension. This is an extension that had no device. Then forward it via Call Rerouting forms to VM.
This does burn a license but any other number you need to do this with, you can simply add keys to the phantom phone and then forward the keys to VM in the call rerouting forms.
If you don't have a spare license to do this with you can find a phone that isn't being used (much) and program the button on page 3 and set it to "no ring".
Ralph