emise extension is on a key system rather than on a single line telephone, it appears on a separate button on one or more of the telephones in the key system. There may be one or many off-premise extensions to the same or different locations. Some local telephone companies have regulations concern- ing off-premise extensions. For example, if you want to have an off-premise extension from a business and have it terminate at a residence, you must already be paying for a separate residence telephone there. Off-premise extension telephone sets are typically analog (such as a plain single line telephone). Digital telephone systems have a distance limitation in terms of how far a proprietary digi- tal telephone can be located from the PBX control cabinet. Some system manufacturers are designing more distributed systems. These control components of the PBX, such as a modular shelf with circuit boards, are still needed a distant location to provide control for the telephones there. An off premise extension is a dedicated circuit. As newer IP (Internet Protocol) based telephone systems de- velop, the ability to have a proprietary multi-line digital telephone at an off-premise location is now possible, using the Internet or a corporate Intranet as the means of connecting to the main office telephone system. Since there are problems with sound quality still being worked out, this capability is not yet widely in use. Foreign Exchange Lines A foreign exchange line is a type of dial tone line that is brought into a PBX from a distant central office or exchange (the exchange is the first three digits of the seven digit telephone num- ber). These three digits, within each area code, are associated with a particular central office and therefore a particular geographic area. (Note: Telephone number portability enables numbers to be moved to the central office of a competing local service provider.) 216 of The Telecom Handbook Figure 10.5 Foreign Exchange Line (N.J. FX from NYC-Based Organization) Ring Ring! Foreign Exchange Line 7