Thank you for providing a link to the documentation.
How do I create a mailbox?
This post[1] says that ARI cannot query app_voicemail.
ari.mailboxes().update("5000", 0, 0);
Updating a mailbox with /mailboxes/{mailboxName}
(which will implicitly create one)
returns mailbox not found
It was my understanding that I could call /mailboxes/{mailboxName} with the update message count arguments and it would update an extension that had its voicemail turned on in my PBX. I now think I am missing a part of how mailboxes are different then voicemails.