EC not only in terms of cost, but in terms of the initial period, for which you pay even if you speak for just one second, and subsequent time period increments. Many of the ILECs and CLECs also sell long distance calls and related services such as Calling Cards. They may also sell Cellular Service and Data Communications Services including circuits connecting you to the Internet. It is likely that many companies selling these different types of telecommunications services were formerly separate compa- nies that have been merged together. Therefore if you want to buy more than one service you may still feel as if you are dealing with separate companies. For example, there are often different The Telecom Handbook 275 UNDERSTANDING THE INDUSTRY sales people and different billing systems for each different type of service. In some cases, these companies have separate divisions that sell business telephone systems called PBXs. They may also sell a service historically known as Centrex which provides PBX ca- pabilities using the functions of the local telephone company's own switching equipment at their central office rather than on the customer's premises. Your local telephone company is also typically responsible for ensuring that your organization is appropriately listed in the white pages and yellow pages directories and in directory assis- tance. With the proliferation of competitors, these listings can no longer be taken for granted and must be regularly checked for accuracy and to be sure that callers can reach you when request- ing directory assistance. The local telephone company owns a number of central of- fice switches (electronic equipment used to route telephone calls to their destination). The central office may be a large or small building depending upon the number of telephone service sub- scribers in the area. In large cities it is likely that you are located within a mile or two from this central office while in more rural areas you may be many miles away. The cables that deliver the local telephone