er for video and audio conferencing, Internet browsing, email, instant messaging, VolP phone calls, and collaboration. (Courtesy Avaya) calendars, address books, and presence indicators from the same interface. Instant messaging is the ability to exchange e-mail in near-real time without typing in an address. Users merely select an icon representing the individual within the community of users to whom the message is intended, type a message, and then click a submit but- ton. Presence indicators let users know when someone within their community is avail- able for real-time or near-real-time messaging. Users can click icons in their e-mail software to indicate their availability. With collaborative software, users can all look at the same file on their individual PC or tablet screens while on an audio conference call or simply speaking together on the telephone. The softphone can also contain or link to corporate directories. Mobility and Single-Number Service Many telephone systems offer a feature by which calls to desk phones also ring cell phones SO that employees can receive workplace calls, even when they are out of the office. In essence, the mobile devices become an extension of the telephone system. Using this feature, callers can reach staff members-perhaps a sales representative who is out on the road-by dialing the same number as the office phone. They don't have to try two numbers or know when the person is out of the office. When the desk Data Centers and IP Private Branch Exchanges phone is called, both the cell phone and the desk phone ring. The displays on the cell phone and the desk phone show the phone number and name of the calling party, if that service is available from the carrier. If the person to whom the call is directed does not answer, the call is forwarded to the telephone system's voicemail. Session Initiation Protocol-Compatible Trunks Session Initiation Protocol (SIP) signaling is a key protocol for connecting IP networks together. It is also used to connect enterprise locations with