 system (either a key system or a PBX whose control cabinet is located on the business' premises). Some tele- phones in businesses or residences are connected via cable in the walls to cable in the street and back to the local telephone com- pany central office where central office-based switching equipment is located. Some common functions that many telephones can perform are found in the chapter on PBX. The Telecom Handbook * 11 THE TELEPHONE Older Style Telephones Still in Use You may hear some of the following terminology referring to the telephone that originated within the Bell System and is still in use today: A single-line telephone such as one you may have in your home is sometimes called a CV or a 500 set (rotary), or a 2500 set (touch-tone). It can be a desk or a wall-mounted model. The older electromechanical multi-line telephones, which are becoming scarce, were called K sets or key sets, referring to the keys or buttons. Each has a red hold button. The KV has six buttons. Then there is a K-10, a K-20 and a K-30, each with that number of buttons. The term call director is also used to refer to a large multi-line telephone. Different geographic areas of the Bell System sometimes used slightly different terms for the same telephone, such as KK6, which was Southwestern Bell's term for New York Telephone's KV. (Note: Southwestern Bell now part of SBC and New York Telephone part of Verizon.) Collectively, this type of electromechanical key equipment was known as 1A2 equipment. The hold button and lights on the phone were controlled by circuit boards in the telephone equip- ment room. Naming of telephones today is even less standard than in the past. Each manufacturer makes up names for its own telephone instruments. Sometimes the names mean something, such as a model number referring to the number of buttons on the telephone. Labels An important aspect of the telephone is how it is labeled. Some manufacturers provide printed labels to indicate the ex- tension number or feature of each button on the teleph