[Serusers] Enforcing known aliases

[Dave]

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.