Newbie Needs Some General Info

Hello everyone, I am looking at installing an Asterisk server in my office to replace our aging Nortel System. I was wondering if it can be configured to have different extentions (or extention trees) along with different initial greetings for each bank of lines that goes into the server, as we have 4 companies operating out of the same location, each one has its calls answered differently and has a number of mailboxes associated with it. Is this possible in one server or do i need multiple servers to do this. Please accept my appologies as I am not up to speed on the lingo so if I used incorrect terms let me know what the correct ones are.