[Users] Checking received contact against location table

[Dimo]

Hi,
I want to tweak my openser setup so it allows calls (INVITES) only
from registered contacts.
I was wondering if there is a way to check the received contact in the
invite message against the location table in mysql database, where i
store the registered users.

I do not want to make additional digest authorization of the Invites.

I see that the registered() function in registrar module has something
like that, but it makes the check on the request uri, not the contact.

Also if someone can think of a better way to allow calls only from
registered contacts I would greatly appreciate any tips.
Thanks.

Best,
D.