[Serusers] Enforcing known aliases

Hi all, I'm trying to find my way around the ser configuration and the sip protocol in general, so please forgive me if this is a faq (I can't find answers in the list archives). I need to enforce the user agents to pick only their registered aliases/usernames instead of being able to pick an arbitrary contact. Ideally, I would like to have an aphanumeric account for each user agent and a fixed numeric alias stored in a database. If the user agent tried to register with another alias/contact instead of the one stored in the database, it should be denied. How do I implement something similar ? Thank you, Dave.