With Jabber, you can bring your Brown phone number with you, even if you're away from your desk! Jabber is a unified communications application for Windows, Mac OS X, and mobile devices. It'll let you make and receive phone calls*, conduct encrypted chats to others who are logged into the system, and search the Brown directory.
* The range of phone numbers you can call on Jabber is the same as on your Brown phone, if you have one. The default is local calls only.