[Serusers] Cisco 2600 SIP dial peers

Hi all, Just a question before I embark on my next SER venture. I now have a couple of Cisco 7960 phones sitting behind fw's and using transverse NAT and SER auth (The Auth working thanks to JAN) @ different homes. All these homes are connected to the internet via ADSL and use DDNS, my SER server sits on the Internet and have a fixed IP and DNS entry. This config works really well. The next step is to add a Cisco 2600 router to the config and hook 4 FXO ports up to our PABX @ work. I only want authenticated users on SER to be able to forward requests to the router, I can tell the router only to accept connections that originated on the SER server. Is this possible on SER. Kind regards, Emil