[Serusers] Question about OutSide Gateways in SER.

Hello again list. :) I have a question related with the authorization of a PSTN Gateway in SER. Suppose that all my calls are forwarded to my PSTN GW. For this i don't need to have my GW registered in SER, i just forward my calls depending on the prefix, and i add a rewritehost("GW.IP") and this works ok. But my GW also have incoming calls form PSTN to my Registered users in SER. So, when a INVITE from my GW arrives to SER, the INVITE is challenge for authorization, since my GW is not even registered or has a username and password defined the authorization fails and therefore the call fails. Is there a way to define my GW in a "Friendly Zone"?? or something like this? to avoid be registered? Thanks in advance Best regards, Ricardo Martinez.