[Serusers] Ser with Asterisk for Voicemail

Hi, I'm deploying a SER + Asterisk architecture, where SER is used to manage acc, users database and sip routing, and Asterisk is used for voicemail and PSTN gateway. How should I edit ser.cfg to make Ser redirect calls to Asterisk when the user doesn't answer his phone or when the user exists but it hasn't any phone registered? If I'm using also Asterisk for Gateway purpose, I guess all calls to be sent to PSTN, Ser redirects them to Asterisk, but it will try to find a mailbox for every number dialled, when it should distinguish calls for PSTN not to be sent to Voicemail. How can Asterisk distinguish between a call sent for voicemail and a call sent by ser to be redirected to PSTN? It depends on the configuration made in ser.cfg? Thanks, Ricardo.