There's lots of things you can do to manage this, but you need to locate the mailbox first. I have a feeling the ring group number is an alternate extension for another mailbox. As I mentioned before, if there's an MWI appearance for the mailbox, try that for the mailbox number. Otherwise you'll have to SSH in, go into the NuPoint text console (type "console" from an SSH session to the MiCollab server), capture a mailbox dump report and search for your known number.