REGISTER and INVITE Auth

Hey guys,

We are deploying ProSBC and wanted to know something. We are using multiple SIP domains in ProSBC and routing the INVITE and REGISTER to specific PBX IP. We also have a default Open NAP that accepts all the SIP traffic. Right now if we have the correct SIP domain in REGISTER, ProSBC forwards that but if we do not then it sends 400 Bad Request. How we can send 401 or 403 Forbidden if REGISTER does not have a SIP domain that is present in ProSBC.

Thank you for help