I'm not entirely sure what you're trying to do here...
So you have a single Unity running behind a single CM and you're wanting to provide support for remote offices... I'm not sure what you mean by a "script for the remote office so PSTN callers...."
You can use call routing rules to get callers from specific destinations or dialed numbers to different call handlers which can play different greetings and offer different one key options for callers. So you could, for instance, setup different inbound numbers for the different remote offices that all hit the one Unity server and it could play different greetings/take different actions based on the dialed number.
Is this along the lines of what you're trying to do?