There are several moving parts here.  So, let me approach this by providing an example that should work and then you can compare notes and see if there is something missing.  I am going to use example patterns to make this cleaner.

1. Voicemail pilot number:

Create a VM pilot number that is guaranteed unique.  For example *091001

2.  Voicemail Profile:

Create a VM profile MyVMProfile that uses the VM pilot number *091001

Modify VM profile to use masks that match your mailbox extensions, like: XXXXXXXXXXX

3.  Route Pattern or Translation Pattern:

Create a route or translation pattern as follows:

- pattern *091001

- translate called:

- Route List: 

- translate calling: XXXXXXXXXX

(NOW, you must be able to guarantee that all calling parties will have either 10-digits or more than 10-digit calling party number digits for this to be useful)

- note: if using a translation pattern, then you will want the translate called to hit a route pattern which points to the SIP trunk.  It may be useful to decouple the digit translation from the route decision if you want to apply different translations based on how calls are routed to your UM system.  THOUGH, if you want things to be uniform then you may want to do the translation on the route pattern directly.

On the phone, you assign the VM profile and modify forwarding to use voicemail (check the "voicemail" box).

HTH.

Regards,
Bill