I'm thinking of adding a separate land phone line for my home office; to date, I've just been passing out my cel number, so as not to have it tied up by the afternoon phone habits

of middle school-age (I'll just say it, sexist though it is . . . ) girls. (You'd think the IM they are so fond of would cut down on this, but you'd be wrong. It's more fun, apparently, to IM *and* chat on the phone
to the same person.)
I'm wondering how extensions work; is that something built into the phone system you purchase, or is it physically extra phone lines you have run to the site? I've read up a little on it, and I'm still confused on this point.
I'd also like recommendations for a good phone setup/system. I'd like to be able to put someone on hold, and send a new call straight to voice mail if I need to, or by default if I don't answer the new incoming call (I can do that on my cel phone, and it's a nice feature.)
I'd like to be able to treat my fax machine as a separate line, but I'm wondering if I can treat it as an extension. At best, I only use it a couple times a week. Possible, or should I add a separate line for that? When I need to be on the phone and doing fax things in the past, I use my cel; the problem is, reception is a little flat right in our house - I sometimes go outside to hear well. I'd like to get away from that if possible, I think it sounds pretty unprofessional.