The development of inexpensive and reliable amplification provided by the introduction of the transistor in the 1960s made practical the design of a dialing system based on the transmission of relatively low power tones instead of the higher-power dial pulses.

Today most telephones have push buttons instead of a rotary dial. Touch-Tone is an optional service, and telephone companies still maintain the ability to receive pulse dialing. Push-button telephones usually have a switch on the base that the customer can set to determine whether the telephone will send pulses or tones.


E. Business Telephones

A large business will usually have its own switching machine called a Private Branch Exchange (PBX), with hundreds or possibly thousands of lines, all of which can be reached by dialing one number. The extension telephones connected to the large business's PBX are often identical to the simple single-line instruments used in residences. The telephones used by small businesses, which do not have their own PBX, must incorporate the capability of accessing several telephone lines and are called multiline sets. The small-business environment usually requires the capability of transferring calls from one set to another as well as intercom calls, which allow one employee to call another without using an outside telephone line.


F. Cellular Telephones

A cellular telephone is designed to give the user maximum freedom of movement while using a telephone. A cellular telephone uses radio signals to communicate between the set and an antenna. The served area is divided into cells something like a honeycomb, and an antenna is placed within each cell and connected by telephone lines to one exchange devoted to cellular-telephone calls. This exchange connects cellular telephones to one another or transfers the call to a regular exchange if the call is between a cellular telephone and a noncellular telephone. The special cellular exchange, through computer control, selects the antenna closest to the telephone when service is requested. As the telephone roams, the exchange automatically determines when to change the serving cell based on the power of the radio signal received simultaneously at adjacent sites. This change occurs without interrupting conversation. Practical power considerations limit the distance between the telephone and the nearest cellular antenna, and since cellular phones use radio signals, it is very easy for unauthorized people to access communications carried out over cellular phones. Currently, digital cellular phones are gaining in popularity because the radio signals are harder to intercept and decode.


Here are 149,084,370 telephone lines in the world, with thousands more being added every day. It's a lot to keep track of...concentration is essential.