Nope.
If the Windows folks are at all using Outlook for even part of what it does, you really need to connect to Windows Server running Exchange Server, possibly SitePoint, and probably not to a Unix mail system (Postfix, Dovecot, etc) running on OS X Server or another Unix or Linux platform. Experienced Outlook users probably won't be very happy with the results with the Unix tools, if you try that. Microsoft really does this business-mail-calendar-AD-integration stuff quite well, but you're buying the full-stack Windows solution to implement all that, or tying into Azure and the Microsoft on-line services.
AFAIK, there's no CalDAV support for Windows with Outlook by default, and various of the add-on CalDAV-capable calendar clients aren't as capable as what Windows provides.
Might also want to check with Microsoft forum, and see if they have specific suggestions for what clients work with CalDAV, too.
iPhones? Sure. They'll work.