ers from the PSTN, such as AT&T in the days of the Bell System, and the PSTN would run a separate pair of wires to the business for each telephone number purchased. Over time, as business services matured, a business service called Centrex emerged. With Centrex, users within the customer's community of telephones could use abbreviated dialing (less than seven digits) to get to other telephones within the community. Special features were added to attract business customers. The customer's telephones were still connected by dedicated wires to the local exchange office. If the customer purchased a telephone number for each telephone, outside callers could reach the customer's telephones di- rectly, a service called direct inward dialing (DID). The customer also could purchase a limited number of telephone numbers and use an operator or second dial tone to reach individual telephones. Each telephone in this arrangement had an extension number, a number not visible to the PSTN. Some business customers chose to purchase only a few numbers from the PSTN and to install a switchboard from which an operator would connect the numbers to tele- phones within the business. This approach used the telephone lines more efficiently, but a human operator was required to connect calls. Jumping forward in time, business customers today still have the option of purchas- ing Centrex services, but many choose to own their own switching equipment in the form of a PBX. PBXs and Centrex remain in heated competition, but the PBX offers as advantages the economies of being able to use fewer connections to the PSTN, the ability to set up private networks, and advanced features not generally available from Centrex. The modern PBX connects to the local exchange office and sometimes directly to in- terexchange carriers as well using groups of trunks. The actual number of trunks used is selected as appropriate to the number and length of calls expected and the cus- tomer's tolerance for blocking, that is, getting a busy signal. The selection of t