I don't know what "all clients" means (are clients your customers? agents? telephone companies? sub-rooms?).
But the usual answer to questions of this sort is:
If the server has TWO network cards, one is usually a private IP (192.168.x.x or 10.x.x.x) and the other card is usually a public IP address. In any event, at some point you will be sending audio packets to someone (agents and/or telephone carriers). The file "sip.conf" has an entry for "externip". This value must be set to the public IP address of the server for any traffic on the public IP address to route correctly.
It is not necessary to have two network cards. It is not necessary to have a public IP directly on the server (either is fine: public or private). In fact, it's not even necessary to have a Static IP. Dynamic works fine. You just have to tell everyone what the new one is when it changes (which in many cases is every six months).
But if you do have a private IP, the externip value in sip.conf must have the public IP that is in use by the router for that network.
So if you have Public IP -> Router -> Private network -> Private IP on ETH0. Then the public IP for that route must be listed in the externip in sip.conf, since eventually that's where all the traffic goes ... AND that's where the return traffic must come back. If that value is not listed in externip, then the traffic will not arrive at that public IP and can not be routed internally to land in the Vicidial server because it never arrived.
Also; the installation guide for Vicibox is NOT the last manual you need. After that installation is complete, you use the Vicidial Manager's Manual for configuration and operations. EFLO.net has it, free and paid versions. While I *highly* recommend the paid version, the free version has enough to get anyone going.
Hope that helped.