nt in the central office looks at the digits dialed and identifies the call as belonging to a particular organization. The central office passes the last three or four digits of the dialed number to the organization's telephone system. The onsite telephone system reads the digits and sends the call directly to the correct telephone. In the case of a T1 failure, calls can be routed on backup individual analog or T1 lines. Circuit-Switched Proprietary PBXs A PBX is an older-type telephone system that uses circuit switching. They are also referred to as proprietary PBXs. Circuit switching saves a path for each voice conver- sation. Although these types of systems are no longer manufactured, many are still in use. Each PBX manufacturer uses proprietary signaling to link telephone and trunk lines to the CPU. Features and peripherals such as contact center software also operate on proprietary signaling. In PBXs that support a single building, the components are generally housed in one cabinet. The main components of a proprietary PBX are the CPU and circuit packs for dial tone, touch-tone, and other signals. Other ports connect to trunks, individual telephones, and voicemail. Value-Added Applications for Telephone Systems VALUE-ADDED APPLICATIONS FOR TELEPHONE SYSTEMS The following value-added services add functionality to telephone systems. Unified Communications (UC) adds efficiency by combining voicemail, e-mail, instant mes- saging, audio and desktop video conferencing, and presence to a single mailbox. Con- tact centers manage large numbers of incoming and outgoing telephone calls, faxes, and e-mail messages. Voice response units enable callers to enter and access com- puter information. UC applications are available on IP-wired phones as well as mobile devices. Integrating Conferencing, Voicemail, Instant Messaging, and E-Mail through UC UC systems are optional features of voicemail systems. They provide the capability to retrieve e-mail and voicemail messages from a single device such as a PC or a touch- tone te