Create an ephone-dn with a dummy extension. The user does not have to associate it with a physical phone. Then call forward all to the voice mail pilot number on the telephony service.
This is an example of the configuration required in the Cisco CallManager Express (CME):
!---This is the dummy extension
call-forward all 6000
!--- This is the voice mail pilot number
On Cisco Unity Express, create a mailbox for the dummy number 1234.
For information, refer to the call-forward all (ephone-dn) command.
Note: This is only an example. Replace with Directory Numbers (DNs) that have not been configured on CME.