Presence
The phone itself doesn't need to be anything special. Though on a previous project at a small biz I went ahead and got Snom 821s which are pretty great and the larger than normal display meant I could retrieve information from their intranet DB when a call came in and show it on the screen (company name, account status etc). They came in at about £160 which wasn't that much more than many dumber IP phones.
But really the intelligence should be on the server-side and it's how you integrate it into the company. For example at this same site I made a natty little addon for Zimbra (zimlet) to display the output from a web service which took XMPP presence, merged with SIP extension status from Asterisk, allowing users to get a realtime view of who was online or busy. Great for users to use their XMPP presence almost as internal tweets (eg: "boarding the plane, back online at 2:30") and you could filter and scroll through that info from within the e-mail client and on the intranet. Overkill for this particular client but very cool nonetheless.
That's the sort of thing that I'd like to see more of in larger companies but they tend to just buy expensive unified comms sytems from big vendors. You don't need clever phones but clever integration. That's where the big vendors make their money as integrating and aggregating data from open source communications software is still fairly painful.