While things can always change in the future, Jabber's parts and pieces are separate components.
Adding multiple line appearances to the configuration obviously took them a bunch of time to manage that, and the UI for it.
Voicemail is its own component and piece, and uses the Unity Connection APIs to display voicemail associated with the credentials you supplied or are configured for you based on how the client is setup. You could build your own CUMI application and achieve more or less the same thing.
That mechanism has no awareness of "lines" as a concept. What'd you have to do with it, given the current arrangement, is have multiple voicemail 'accounts' setup for each of those lines, so you could view the mailbox.
Since CUC can send message counts, hopefully in the future you would at least be able to see mail is waiting, then select that line to call the voicemail pilot. Today you have no idea.
As others have suggested there are a number of ways to tackle this, mainly revolving around distribution. There are some games you can play with alternate extensions but this doesn't help you share that mailbox. You can also supply links in a custom tab to the web inbox. There's a graphical API which can display the MWI status of the mailbox with an icon, but that takes a bit of work to put together obviously.
This is a conundrum that is best cracked if you don't get down this road to begin with, that the system has moved over the last decade from "license units" and phone objects to user centrality. Re-designing your dialplan and station layout is difficult, but, spending the pain to disentangle shared lines/mailboxes from your workflow can pay off later both with services such as this, but Webex Teams or other user-centric collaboration services.
