With advent of increasingly new ways of communicating, like mobile, chat , e-mail et al there is a need to manage the interface across all these media. Let us think it this way. If you are at office you want to be contacted at office number on mobile, if you are at home you want to be contacted at your Home computer through E-Mail. So the result is there needs to be a way to manage your contact through various interfaces and gadgets.
This is achived through Unified Communications. What with Microsoft having a dedicated server for it although an integrated solution is still a distant dream.