[Users] Checking received contact against location table

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.