[Serusers] Generic Question

Hi all. I'm new to this list and I have several doubts about how SER works. I have been working with H323 til last week ( installing and configuring a GNUGK ) in my work and now I have to face SIP. It would be perfect if someone could help me just answering these questions: 1.- Once SER is installed with a public IP of my network, can I enforce all clients inside ( with private IP ) to go through the SER server always ? I mean, calls generated inside my network to outside and the calls generated from outside clients ( who are not registered to my SER server ) to inside clients. I don't want anyone from inside to establish a call without using the SER server and viceversa. 2.- An outside client who is only registered with Microsoft Registar ( .NET account I think ), could be reached from a client who is only register with "my" SER server ? And what about the other way round ? 3.- Does anyone know any free SIP client with videoconference support for Linux / Windows ? ( Not only VoIP, audio + video ). I have been using the free account that IPTEL provides and I liked it very much. I had no problem with nat ( if both parts are registered in the IPTEL server ) and it seems solve a lot of problems I had with H.323. Thank you very much in advanced: Kiko