 you a dial tone. It sends the number of the telephone to be called. It indicates the progress of your call through tones: ringing, busy, etc. It rings to alert you to an incoming call. It transforms your speech into electrical signals for transmis- sion to a distant point and translates the electrical signals it receives back into the human voice for you to hear. It automatically adjusts for changes in the power supplied to it. When you hang up, it signals the telephone system that you are finished. The telephone is also referred to as a telephone instrument, station or set. There are single-line telephones and multi-line tele- phones (also called multi-button telephones). These telephones can be analog or digital and you cannot always tell which is which by appearance. Telephones work with outside telephone lines from the local telephone company or as extensions from a business telephone system (PBX or Key System). The telephone is designed to operate under a wide range of electrical, mechanical and acoustical conditions. Some of the de- sign parameters are dictated by human factors such as sound levels of the human voice and handset dimensions (the handset is also The Telecom Handbook * 3 THE TELEPHONE called the telephone receiver). Some parameters are historical carryovers such as ringing voltage and frequency. Others, such as the minimum line current for satisfactory carbon transmitter and relay operation, are dictated by the physical properties of the materials used in the telephone. Telephones in use today are of different vintages. The more recently manufactured telephones substitute microphones for some of the materials such as carbon transmitters still found in many older telephones in use. The following pictures show a single-line telephone (Figure 1.1) and a multi-line telephone (Figure 1.2). As you can see, they vary somewhat in appearance, but have some things in common. Figure 1.1 - Traditional Single Line Telephone (Also called 2500 Set) You speak into the Transmitter You listen through the Recei