[Serusers] Multi-Domains / RADIUS Multi-Realms

I'm trying to setup something like virtual PBX for a few of my client's offices. I was reccommended on this list to look into multi- domains and multi-realms in order to allow, for example, two offices to have the same numbering sequence for their telephone extensions (that is, two distinct clients could have extensions 101, 102, etc) so that when they call the extension number, SER will route the call to the extension within that office. I haven't been able to find much info on exactly what I wanted to do so if anyone can point me in the right direction, I would appreciate it. I know I could do this with Asterisk using contexts, but I would rather stay within the SER environment. The other requirement is that calls placed to the outside world (meaning to external numbers whether through PSTN or VoIP) should show the office's telephone number as the caller ID instead of the extension calling. So, basically, when calling extension to extension, it should show the extension number as the CLID, but when calling outside numbers, it should show the office's main number as the CLID. Does anyone know where I can find more info about this or even provide some sample configs? Thanks, Waldo