Several mailboxes, one extension

I’m trying to set up voicemail where each user has their own mailbox, but only one extension is used to access them or create them. Ideally, a user would be assigned the next mailbox number and be prompted to enter a password. Is there a way to do this?

I’m able to have one extension access all of the mailboxes by sending an incoming route directly to voicemail with a feature code, but I haven’t found a way to let users create a mailbox which isn’t associated with the extension they’re currently using.

Maybe there’s a way to get the next unused mailbox and send a user directly to the change password prompt of the voicemail menu?

I’m using Asterisk 1.8.11.