I have an Avaya Lucent Partner 18D phone system. My problem is that one extension (#10) is saying that the mailbox is full when customers call in, but there are no messages in the mailbox. I have reinitialized mailbox, powered down the system, and reassigned the mailbox to the same extension number. Nothing has worked and I am at a loss as to what to do.

which VM ? try deleting and rebuilding the MB

Not sure how to do that. We don't have a manual so I have no way to look it up.

I would suggest its time to get a tech in . click on the installer link at the top of the page here and see who is in your area .
you could also contact Mike Daniels , a member here for remote support . . Mikes the guy I go to for Avaya quesions

Avaya Lucent Partner 18D is not the phone system, it's the model of the phone sitting on your desk. We need to know what Partner system you have and what messaging system as there are several possibilities for both. It's quite possible that calls are being sent to the call answer service and that's the mailbox that is full. But again, not all Partner messaging systems have call answer service.

-Hal

As Hal said it my be that the call answer service is set for 10 depending on what message system you have, try 9991 and see if that's where the messages are.