Log into switch and go to Extensions. Find the mailbox extension and click on it.    Go to the Voicemail and change Mailbox Type to Distrib. Under List Member you will need to click on the drop down window and select the extension you want to add then click the plus sign on the right side of the List Member window.       Once change is made hit apply

Save and make sure to notate the ticket number and what you did.

For in-depth over view of extension