[Serusers] Routing - Looking up database table for subscriber accounts not registered at the moment

Hi all i just started working with SIP and i'm currently trying to implement the routing logic as needed in my setup. Doing this i came across a requirement i couldn't find a solution for. Maybe someone of you may help me? I have the following situation: I have users with (only) e164Numbers as usernames in my SIP domain and i have a PSTN Gateway behind which i may reach also e164Numbers. Now, i have a subscriber base in my domain where not all of the subscribers with a e164Number may be registered at a certain moment (but still have a valid account and phonenumber). When i get a call at such a moment, i will try to look up the callee username in the location database. If i find one, fine. If not, there are two possibilities left - first, it could be a user only reachable in the pstn, so route to the pstn gateway or second it might be a user in my domain with a valid account but not registered at the moment, so reject or redirect to a voicemail or what ever. If I would work with alphanumerical numbers only within my SIP cloud, i knew how this could be done - all numerical numbers automatically would be PSTN numbers. But how can this be done in my setup? Thanks a lot for your help and kind Regards Frank