Yes, you can do this with Phone manager pro or press transfer # and the extension number and transfer.  Or if you want a button you can create a short code of something like this.

Short Code: *56
Number: directvm
Feature: Voicemail Node
Dial: 0

Then you need to create a place in the voicemail shortcodes called directvm and in you start point use the leave mail option with $KEY in the specific tab.  Record a greeting such as "Enter extension number" and let it be interupted by touchtones.

Or evan simpler

Short Code: #7xx
Number:     #7n

Feature: Voicemail Collect

Dial # Extension number instead of just ext number and will go to mailbox

Soopa Doopa Intergalactic IP Office installation mechanic and configuration corruptor from way back.

I like IPAlot answer


thanks y'all

IPAlots Answer should read

SC #7xx
Number "#<Extn Name>"
Feature Voicemail collect

Voicmail requires the usere Name NOT Extn number

IPAlots solution may work on some releases of firmware but it cannot be garanteed to continue to work