It would be nice if the module would be able to manage voicemail boxes too.
Is there interest? I'd look into it, provide a patch.
I'd keep it simple but enhanceable, initially only supporting my use-case,
i.e. being able to configure voicemail boxes, and sending out
mails informing the owner of the boxes. Storing them in imap, etc.
I'd leave that for others ;)
I'd take a similar route like the other parts of the module are
managed. Just using easy template to configure the stuff.