Telephone

A telephone, or phone, is a telecommunications device that converts sound into electronic signals suitable for transmission via cables or other transmission media over long distances.

It was first patented by Alexander Graham Bell in 1876. The telephone was the first device in human history that enabled people to talk directly with each other across large distances. It rapidly became indispensable to businesses, government, and households, and it is now among the most widely used appliances in the developed world. The word telephone has been adapted into the vocabulary of many languages. It is derived from the Greek: far and voice, together meaning distant voice.

The essential elements of a telephone are a microphone (transmitter) to speak into and an earphone (receiver) which reproduces the voice of the distant person. In addition, most telephones contain a ringer which produces a sound to alert the user when a telephone call is coming in, and a dial used to enter a telephone number when initiating a call to another telephone.

Until approximately the 1970s most telephones used a manual rotary dial, which was superseded by the modern Touch-Tone push-button dial, first introduced by AT&T in 1963. The receiver and transmitter are usually built into a handset which is held up to the ear and mouth during conversation. The dial may be located either on the handset, or on a base unit to which the handset is connected by a cord containing wires.

A landline telephone is connected by a pair of wires to the telephone network, while a mobile phone, such as a cellular phone, is portable and communicates with the telephone network by radio transmissions. A cordless telephone has a portable handset which communicates by radio transmission with the handset base station which is connected by wire to the telephone network.

The transmitter converts the sound waves to electrical signals which are sent through the telephone network to the receiving phone. The receiving telephone converts the signals into audible sound in the receiver, or sometimes a loudspeaker. The telephone network, consisting of a worldwide net of telephone lines, fiberoptic cables, microwave transmission, cellular networks, communications satellites, and undersea telephone cables connected by switching centers, allows any telephone in the world to communicate with any other. Each telephone line has an identifying number called its telephone number.

Although originally designed for simple voice communications, most modern telephones have many additional capabilities. They may be able to record spoken messages, send and receive text messages, take and display photographs or video, play music, and surf the Internet. A current trend is phones that integrate all mobile communication and computing needs; these are called smartphones.

The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network.

A hardware-based IP phone, with touch-tone dialing Internet Protocol (IP) telephony is a disruptive technology that is rapidly gaining ground against traditional telephone network technologies. IP telephones have two notable disadvantages compared to traditional telephones. Unless the IP telephone's components are backed up with an uninterruptible power supply or other emergency power source, the phone will cease to function during a power outage as can occur during an emergency or disaster, exactly when the phone is most needed. A second distinct problem for an IP phone is the lack of a 'fixed address' which can impact the provision of emergency services such as police, fire or ambulance, should someone call for them.

By the end of 2009, there were a total of nearly 6 billion mobile and fixed-line subscribers worldwide. This included 1.26 billion fixed-line subscribers and 4.6 billion mobile subscribers.