Assume you need a single mailbox for extensions 5001 and 5003.
In CUCM go to> Feature> Voicemail> Voicemail Profile and create a new Profile called XXXX (whatever name you want) with a description called "XXXX username Mailbox" the Voice Mail Box Mask will be 5001> Save.
Then via CUCM go to the users Phones> under Directory Number Config for 5003> Voicemail Profile and change to XXXX (created previously in above steps)>Update>Reset.
Now when a call routes via forward to Unity Connection it will reach the Mailbox @ 5001.
Or in Unity Connection set up Alternate Extension so that 5003 is an Alternate Extension for 5001.
Hope this helps.
Regards,
Anoop
