Any good phone system on a Windows Environment?

OCS is great if you can afford it. Remember that people wanting the phone to just act like an old phone system may not go for it. It really thinks in a new paradygm. If you cannot afford on premise OCS checkout hosted OCS. I've written an article going over some Hosted options.

You could take a peek at pbxnsip for Windows. This is a very stable IPPBX for Windows. pbxnsip has a ton features to match older, traditional pbx's and integrates to Exchange and OCS. It does not have AD integration. At the moment 3CX will import aD users but nothing more.

I think Adtran has a windows product (forget name) that does have AD integration if that is a deal killer. My impression is that it was a little lean on features (no blf, etc)

On 3CX: I've written a book on 3CX and spent multiple 1000's of posts helping people with it and 3CX has User Interface nailed. But to my disappointment my personal experience has been that it isnt the "set and forget" level I'd expected from a pbx. Our consultancy uses & recommends pbxnsip for Windows PBX. It has all those features traditional pbx'ers throw at you and runs like a rock.

So you can pick: Easy to use, lighter on features, less rock solid, cheaper= 3CX. Every feature you can think of, extremely rock solid, bigger learning curve, not bargain basement=pbxnsip. Move into the future and lay out some cash=OCS.

If you are looking at IP phones in a microsoft environment you sould consider OCS. If all you want is a phone system OCS does a pretty good job. as well as just phone you get conferncing and im/presence capabilities integrated into the product (Eg if I call you via pots line you might respond to me via cell phone, IM/VOIP or regular handset phone on yhour office desk, or your phone at home, depending on your presence and availabilty schedule) It really shines however when you pair it with exchange 2010. Now instead of just a phone system you get unified communications. the downside to all these wizbang features is that it's relatively complicated to set up (and I'd say that with the demise of responsepoint, that's across the board whether it's asterisk, avaya, or cisco)

Costwise depends on how much Microsoft you already have. If you are heavily invested in microsoft tech already, talk to your favorite reseller then get them to get you in contact with your microsoft rep to work on additional discounts. I also think that there are implementation dollars available from microsoft so that the implementation work can be done for free (or at least at a really really reduced price) from a microsoft certified partner.

We use a product called Swyx. I'm not sure how you would classify "Not too expensive", but we've scaled it over multiple sites and to about a hundred users. It's got fantastic support, and was very reasonably priced compared to Cisco Callcentre (and as someone who used to work for Cisco's global contact centre, you do NOT want to be calling Cisco's AVVID support team on a weekend or after-hours, I saw the wait times for their queues).

Anyhow, it's got Outlook, Exchange and AD integration, a very good, skinnable soft-client and supports standard SIP clients and numerous purpose-designed physical phones. You can control someones phone via Terminal Server, so if you have a TS app that can initiate a phone call you can do that too.

I strongly recommend getting a demo and some pricing to see if it fits your budget. From memory one of our clients spent about $AUD50k rolling it out to a huuge company, and another client spent well under $AUD10k rolling it out to a single office.