I'd love to be able to recommend Cisco... and actually, I can... if you've got the budget for it..
Cisco will do everything you're asking for, but you're looking at a pretty extensive system for only a small number of users.. You're going to buy big servers, without a large user population to "divide it by", giving you a higher price per handset...
Cisco scales better as you get larger. For the smaller installs, I'd be inclined to recommend call manager express, but you've listed a couple features that I'm not sure you could do w/ Express...
Especially where you say the customer service attendant can see who's doing what.. I presume you want it to actually look at the Outlook calendar to do that? For Cisco, that's an add-on called "Presence", which runs on a separate server. Everything else you mention can be done on one "Business Edition" server, which is a combination Call manger and Unity... If all you want is to know if someone is on the phone, then you can probably do it with a busy lamp field, either software or hardware, with the CM or CME.
The other tough thing may be to do the remote phones.. it's EASY, if you use soft phones, because then the client PC just has to VPN from anywhere, and it works. If you want a hard-phone to work from remote, you either have to have a site to site VPN setup, so that the phone can be on a VPN'd network segment, *OR* you can use what they call the PhoneProxy, which is a feature of the Cisco ASA, which allows a phone to be encrypted over the public Internet and pass through the firewall to the call manager. That's licensed by the session on the ASA, so it's an additional software cost..
I will tell you that Cisco is EXTREMELY flexable, extendable, expandable, and you're going to be able to have continued feature growth with new versions and add-on software better than any "smaller" pbx, but if you're just looking for basic phones w/o looking at features and connectivity into the future, Cisco isn't going to win on price alone...
On the otherhand, *IF* your business is going to grow, and have remote offices, the expansion is going to be where you're going to save... Once you buy the infrastructure, starting a new office is ALMOST as cheap as buying the phones... You're already going to have to buy a switch and router for the site, so it's incrementally more expensive to add the PoE and voice ports to those components, and the only other thing you need are the phones... SO, again, if you're going to grow, the Cisco system gets VERY cheap per handset as you install more remote offices or more handsets..
-Steve
