What infrastructure and endpoints are you using, as well as software versions for each?
I know you're using an MX800 from your other discussion here and I presume have the endpoint configured per the presentation slide desk I linked to. Did you check the other pages, regarding other required settings to get it working?
Required servers, versions, and special settings if any.
Beyond trying to help make sure you have the correct server versions and settings, as I don't have CUCM, I can't help do an actual test.