Unified Communications at Microsoft supports over 40,000 users. Multiple communications tools such as e-mail, Instant Messaging, voice mail, voice, and presence are delivered in a unified experience which is available anywhere. Because multiple and disparate communication systems have been consolidated and in sourced onto a single solution, the ability to manage and measure the environment is enhanced.

http://edge.technet.com/Media/Unified-Communications-Delivers-a-Scalable-Consolidated-and-Manageable-Communication-Experience/