sounds to me like the mailbox is set up as a hotel Mailbox and isnt "checked in".. if you call the room it forwards t ovoicemail and you hear "You are being transferred to... the operator" that means the mailbix is in a hotel room checked out status... you need to either set up a PMS interface to the voicemail or log into a front-desk type mailbox and check the guest room mailbox in....
this feature is done so that it eliminates hotel guests getting stale messages from previous guests when they check in
-Christopher