If you don’t want to run an MS Exchange server, but you do want to have the groupware functions such as sharing calendars, emails, notes, contacts, etc. there are a few semi open alternatives you can try which also support phone synchronisation.
Most support webbased functionality and allow you to attach any client to it, some come with their own suite of clients.
But if you want to use your Outlook client, all of them require you to buy a connector.
Unison has a client and server setup which also includes IM and a PBX for IP telephony.
Most work quite a bit like Outlook, but Citadel is totally different, out of the box.