[Serusers] Routing

Hi! IŽm trying to make a dialing plan with ser. So i have configurated the ser.cfg in order to route for some especific destinations to other SIP proxies. But some of them need digest authentication and I need to do it on the ser. The scenario would be like this: 1- A sip client, which is registered in my ser proxy, tries to make a call to a destination that doesnŽt belong to my network. 2- So ser should route this call to a third party proxy, which provides call termination (I have an account in this third party proxy). 3- When ser routes the call to that proxy, this response with challenge for authentication. The question would be: How do I reply to that challenge with ser ? Is it posible to do this with ser ? Thanks a lot ! Regards, Gabriel