ephone company central office. The above illustration show Ext. 2480 placing an internal call to Ext. 2411. (If someone from the outside dials 883-2411, the call will ring directly to that telephone.) CHAPTER 10 The idea behind Centrex is that your organization does not need to buy a PBX. Instead, the switching of your calls takes place at the tele- phone company central office, with the telephone extensions coming out to your premises through cables under the street or overhead (most typically delivered using T-1 circuits). You use system features such as call transfer and call conference, which are capabilities of the central office switch. It is possible to use almost any telephone system with Centrex service, but some make more sense than others. The best solution is usually to purchase the telephones that are made to work with the central office switch. Northern Telecom makes a central office switch called the DMS-100. If your Centrex service is delivered from the DMS-100, you may want to buy Northern Telecom telephones to realize the maximum capability for using the system features. Lucent Technologies (formerly AT&T) also makes central office switches, as do Siemens, NEC, Ericsson and others. Centrex is particularly good for providing telephone service to multiple locations of the same company or organization within the geographic area served by the central office. For example, a municipality or a university with multiple buildings may benefit from Centrex service. In cities, Centrex typically serves a smaller geographic area than in areas less densely populated. With Centrex service, each telephone in the system has a sepa- rate telephone number and carries extra monthly charges for certain system features. It tends to be a more costly system, over time, than an on-site PBX, but some organizations like the idea of not having to manage their own PBX. If you decide to use Centrex, you may not have as much control to administer and troubleshoot the system as you would have with a PBX on your premises. Also, 