User Without Mailbox?

I configured a new user for my PBX today. His voicemail box works and we checked it by using it and transferring massages to his inbox. But the CLI keeps repeating this message:

[Sep 28 19:39:08] NOTICE[960]: chan_sip.c:15746 handle_request_subscribe: Received SIP subscribe for peer without mailbox: 3001

Why would Asterisk report that this SIP subscriber has no mailbox when he clearly does?

I’m fairly sure it means the phone does not know about the mailbox.