Microsoft's new Instant Messaging and real-time communications client, Microsoft Communicator 2005 (previously codenamed Istanbul), will be a "client of choice" replacement for Windows Messenger in the new version of the Office Live Communications Server (LCS). Windows Messenger however, will be still be continously developed and shipped with the operating system.

Communciator 2005 goes beyond IM handling a variety of functions based on "presence," the notion advocated by Microsoft and others of software being able to intelligently route communications among various applications and devices. For example, Istanbul reads calendar information from Outlook to decide whether an incoming call should go to someone's desktop or mobile phone.